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