needed on windows for py>=3.8

update classifiers in setup.py

When a package is installed in development mode by `pip install -e`, it
calls `python setup.py develop` under the covers

    https://setuptools.readthedocs.io/en/latest/setuptools.html#development-mode

However it currently skips build_dso in that mode which leads to build
failure if anything - e.g. extension - is depending on a dso:

    + pip install -v -e .
    Created temporary directory: /tmp/pip-ephem-wheel-cache-OJgoBO
    Created temporary directory: /tmp/pip-req-tracker-CrRj5z
    Created requirements tracker '/tmp/pip-req-tracker-CrRj5z'
    Created temporary directory: /tmp/pip-install-hiu8rq
    Obtaining file:///home/kirr/src/tools/py/pypa/setuptools_dso/example
      Added file:///home/kirr/src/tools/py/pypa/setuptools_dso/example to build tracker '/tmp/pip-req-tracker-CrRj5z'
      Running setup.py (path:/home/kirr/src/tools/py/pypa/setuptools_dso/example/setup.py) egg_info for package from file:///home/kirr/src/tools/py/pypa/setuptools_dso/example
        Running command python setup.py egg_info
        running egg_info
        creating src/dsodemo.egg-info
        writing src/dsodemo.egg-info/PKG-INFO
        writing top-level names to src/dsodemo.egg-info/top_level.txt
        writing dependency_links to src/dsodemo.egg-info/dependency_links.txt
        writing manifest file 'src/dsodemo.egg-info/SOURCES.txt'
        reading manifest file 'src/dsodemo.egg-info/SOURCES.txt'
        writing manifest file 'src/dsodemo.egg-info/SOURCES.txt'
      Source in /home/kirr/src/tools/py/pypa/setuptools_dso/example has version 0.1, which satisfies requirement dsodemo==0.1 from file:///home/kirr/src/tools/py/pypa/setuptools_dso/example
      Removed dsodemo==0.1 from file:///home/kirr/src/tools/py/pypa/setuptools_dso/example from build tracker '/tmp/pip-req-tracker-CrRj5z'
    Installing collected packages: dsodemo
      Running setup.py develop for dsodemo
        Running command /home/kirr/src/tools/py/pypa/setuptools_dso/env/bin/python -c "import setuptools, tokenize;__file__='/home/kirr/src/tools/py/pypa/setuptools_dso/example/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" develop --no-deps
        running develop
        Checking .pth file support in /home/kirr/src/tools/py/pypa/setuptools_dso/env/lib/python2.7/site-packages/
        /home/kirr/src/tools/py/pypa/setuptools_dso/env/bin/python -E -c pass
        TEST PASSED: /home/kirr/src/tools/py/pypa/setuptools_dso/env/lib/python2.7/site-packages/ appears to support .pth files
        running egg_info
        writing src/dsodemo.egg-info/PKG-INFO
        writing top-level names to src/dsodemo.egg-info/top_level.txt
        writing dependency_links to src/dsodemo.egg-info/dependency_links.txt
        reading manifest file 'src/dsodemo.egg-info/SOURCES.txt'
        writing manifest file 'src/dsodemo.egg-info/SOURCES.txt'
        running build_ext
        Traceback (most recent call last):
          File "<string>", line 1, in <module>
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/example/setup.py", line 26, in <module>
            x_dsos = [dso],
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools_dso/__init__.py", line 37, in setup
            _setup(**kws)
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools/__init__.py", line 145, in setup
            return distutils.core.setup(**attrs)
          File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
            dist.run_commands()
          File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
            self.run_command(cmd)
          File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
            cmd_obj.run()
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools/command/develop.py", line 38, in run
            self.install_for_development()
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools/command/develop.py", line 140, in install_for_development
            self.run_command('build_ext')
          File "/usr/lib/python2.7/distutils/cmd.py", line 326, in run_command
            self.distribution.run_command(command)
          File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
            cmd_obj.run()
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools_dso/dsocmd.py", line 395, in run
            _build_ext.run(self)
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools/command/build_ext.py", line 78, in run
            _build_ext.run(self)
          File "/usr/lib/python2.7/distutils/command/build_ext.py", line 340, in run
            self.build_extensions()
          File "/usr/lib/python2.7/distutils/command/build_ext.py", line 449, in build_extensions
            self.build_extension(ext)
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools_dso/dsocmd.py", line 406, in build_extension
            self.dso2lib_pre(ext)
          File "/home/kirr/src/tools/py/pypa/setuptools_dso/env/local/lib/python2.7/site-packages/setuptools_dso/dsocmd.py", line 146, in dso2lib_pre
            raise RuntimeError("Unable to find DSO %s needed by extension %s"%(dso, ext.name))
        RuntimeError: Unable to find DSO dsodemo.lib.demo needed by extension dsodemo.ext.dtest

-> Fix it by making build_ext to depend on build_dso.

In the next patch we will need to inject build_ext -> build_dso
dependency and will use this infrastructure.

- they are not used in the example
- they are injected as command classes by setuptools_dso.setup itself

Only need to include @loader_path/. if a dependency
is in the same directory.

@loader_path prefix requires dependent libraries to be in the same
directory or to use install_name_tool to change the name after linking.
With this fix, linker's -rpath can be used instead.

Which requires freeze_support()

Try to avoid problems only detectable in tag builds.

Expedite recommendation by Cython authors to distribute
generated c/c++ source files.

package_dir needs to be taken into account for inplace builds, because
then inplace directory of a package != directory of the package when it
is installed. For example setuptools_dso itself uses
`package_dir={'':'src'}` in its setup.py and this way for inplace build the
directory of setuptools_dso package is not setuptools_dso/ but
src/setuptools_dso/ instead.

However `build_dso --inplace` was installing built libraries without
taking package_dir into account. For example with amended example/ test,
libdemo.so was installed into dsodemo/lib/libdemo.so instead of
src/dsodemo/lib/libdemo.so, while dtest.so was correctly installed into
src/dsodemo/ext/dtest.so . As the result it was not working:

    + python -m dsodemo.cli
    Traceback (most recent call last):
      File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
        "__main__", fname, loader, pkg_name)
      File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
        exec code in run_globals
      File "/home/kirr/src/tools/py/pypa/setuptools_dso/example/src/dsodemo/cli.py", line 15, in <module>
        from .ext import dtest
    ImportError: libdemo.so.1.0: cannot open shared object file: No such file or directory
    .../setuptools_dso$ ll example/
    итого 16
    drwxr-xr-x 4 kirr kirr 4096 дек  4 18:12 build
    drwxr-xr-x 3 kirr kirr 4096 дек  4 18:12 dsodemo
    -rwxr-xr-x 1 kirr kirr  639 дек  4 18:12 setup.py
    drwxr-xr-x 3 kirr kirr 4096 дек  4 18:06 src
    .../setuptools_dso$ ll example/dsodemo/lib/
    итого 136
    -rwxr-xr-x 1 kirr kirr 68888 дек  4 18:12 libdemo.so
    -rwxr-xr-x 1 kirr kirr 68888 дек  4 18:12 libdemo.so.1.0
    .../setuptools_dso$ ll example/src/dsodemo/ext/
    итого 124
    -rwxr-xr-x 1 kirr kirr 119000 дек  4 18:12 dtest.so
    -rw-r--r-- 1 kirr kirr      0 июл 30 21:37 __init__.py
    -rw-r--r-- 1 kirr kirr    110 дек  4 18:12 __init__.pyc
    .../setuptools_dso$ ldd example/src/dsodemo/ext/dtest.so
            linux-vdso.so.1 (0x00007fffae3fd000)
            libdemo.so.1.0 => not found
            libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f1c3bf20000)
            libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f1c3bd9d000)
            libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f1c3bd83000)
            libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f1c3bd62000)
            libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f1c3bba1000)
            /lib64/ld-linux-x86-64.so.2 (0x00007f1c3c149000)
    .../setuptools_dso$ objdump -x example/src/dsodemo/ext/dtest.so |grep PATH
      RUNPATH              \$ORIGIN/../lib:$ORIGIN/../lib

-> Fix it by taking package_dir option into account in --inplace dsocmd handler.

Note1: The code to do so mimics build_ext from setuptools:

    https://github.com/pypa/setuptools/blob/v42.0.2-0-g1d03fdc9/setuptools/command/build_ext.py#L89-L107

Note2: Sources of Extension and DSO are specified with full path from project
top-level, because it already works this way for unmodified Extension even in
presence of package_dir.

Don't allow one to pass into setuptools_dso.setup() e.g. build_ext that
does not inherit from setuptools_dso.build_ext.

Fixes: 67d717a6 (wrapper setup don't clobber)

The code was trying to create solibbase symlink pointing to -> outbaselib, e.g.

    libdemo.so.1.0 -> build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0

However if the source file in worktree already exists, symlinking was
failing with an error. Fix it by making sure that source file does not
exist before calling os.symlink.

Also there was related thinko in log.ingf describing who points to
who/where, which is corrected along the way.

Without the fix newly added test fails as

    ...
    running build_dso
    Building DSOs
    building 'dsodemo.lib.demo' DSO as build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0
    creating build
    creating build/temp.linux-x86_64-2.7
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DBUILD_FOO -c foo.c -o build/temp.linux-x86_64-2.7/foo.o -DALL -DISC
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DBUILD_FOO -c bar.cpp -o build/temp.linux-x86_64-2.7/bar.o -DALL -DISCXX
    cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
    creating build/lib.linux-x86_64-2.7
    creating build/lib.linux-x86_64-2.7/dsodemo
    creating build/lib.linux-x86_64-2.7/dsodemo/lib
    x86_64-linux-gnu-g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -Wl,-z,relro -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security build/temp.linux-x86_64-2.7/foo.o build/temp.linux-x86_64-2.7/bar.o -o build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0 -Wl,-h,libdemo.so.1.0
    symlink libdemo.so.1.0 -> build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0
    creating dsodemo/lib
    copying build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0 -> dsodemo/lib
    copying build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so -> dsodemo/lib
    running build_dso
    Building DSOs
    building 'dsodemo.lib.demo' DSO as build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DBUILD_FOO -c foo.c -o build/temp.linux-x86_64-2.7/foo.o -DALL -DISC
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DBUILD_FOO -c bar.cpp -o build/temp.linux-x86_64-2.7/bar.o -DALL -DISCXX
    cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
    x86_64-linux-gnu-g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -Wl,-z,relro -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security build/temp.linux-x86_64-2.7/foo.o build/temp.linux-x86_64-2.7/bar.o -o build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0 -Wl,-h,libdemo.so.1.0
    symlink libdemo.so.1.0 -> build/lib.linux-x86_64-2.7/dsodemo/lib/libdemo.so.1.0
    error: [Errno 17] File exists

It is used in several places and will be used more in the next patch.