setup: Switch building of Python extensions to Pygolang

Soon we are going to split virtmem code into its own DSO to which
bigfile extension will link. As plain setuptools does not support such
dynamic linking, we are going to use setuptools_dso instead. But more:
some of our upcoming extensions and DSOs will need to use Cython and C++
parts of Pygolang. Prepare that and use Extensions and DSO from
golang.pyx.build to support that right from the start.

pyproject.toml

+[build-system]
+requires = ["pygolang[pyx.build] >= 0.0.8"]

setup.py

@@ -17,9 +17,9 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-from setuptools import setup, Extension as _Ext, Command, find_packages
+from golang.pyx.build import setup, DSO as _DSO, Extension as _PyGoExt, build_ext as _build_ext
+from setuptools import Command, find_packages
 from setuptools.command.build_py import build_py as _build_py
-from setuptools.command.build_ext import build_ext as _build_ext
 from pkg_resources import working_set, EntryPoint
 from distutils.errors import DistutilsExecError
 from subprocess import Popen, PIPE
@@ -29,7 +29,7 @@ import sys
 
 
 # _with_defaults calls what(*argv, **kw) with kw amended with default build flags.
-# e.g. _with_defaults(_Ext, *argv, **kw)
+# e.g. _with_defaults(_DSO, *argv, **kw)
 def _with_defaults(what, *argv, **kw):
     kw = kw.copy()
     kw['include_dirs'] = [
@@ -39,21 +39,26 @@ def _with_defaults(what, *argv, **kw):
             './3rdparty/include',
         ]
 
-    ccdefault = [
-        '-std=gnu99',           # declarations inside for-loop
-        '-fplan9-extensions',   # anonymous-structs + simple inheritance
-        '-fvisibility=hidden',  # by default symbols not visible outside DSO
-
-        # in C99 declaration after statement is ok, and we explicitly compile with -std=gnu99.
-        # Python >= 3.4 however adds -Werror=declaration-after-statement even for extension
-        # modules irregardless of their compilation flags:
-        #
-        #   https://bugs.python.org/issue21121
-        #
-        # ensure there is no warnings / errors for decl-after-statements.
-        '-Wno-declaration-after-statement',
-        '-Wno-error=declaration-after-statement',
-    ]
+    ccdefault = []
+    if kw.get('language') == 'c':
+        ccdefault += [
+            '-std=gnu99',           # declarations inside for-loop
+            '-fplan9-extensions',   # anonymous-structs + simple inheritance
+
+            # in C99 declaration after statement is ok, and we explicitly compile with -std=gnu99.
+            # Python >= 3.4 however adds -Werror=declaration-after-statement even for extension
+            # modules irregardless of their compilation flags:
+            #
+            #   https://bugs.python.org/issue21121
+            #
+            # ensure there is no warnings / errors for decl-after-statements.
+            '-Wno-declaration-after-statement',
+            '-Wno-error=declaration-after-statement',
+        ]
+    else:
+        ccdefault.append('-std=gnu++11')    # not c++11 since we use typeof
+
+    ccdefault.append('-fvisibility=hidden')  # by default symbols not visible outside DSO
 
     _ = kw.get('extra_compile_args', [])[:]
     _[0:0] = ccdefault
@@ -63,7 +68,7 @@ def _with_defaults(what, *argv, **kw):
     # python extensions cannot be built with -Wl,--no-undefined: at runtime
     # they link with either python (without libpython) or libpython. linking
     # with both libpython and python would be wrong.
-    if 0:
+    if what == _DSO:
         lddefault.append('-Wl,--no-undefined')  # check DSO for undefined symbols at link time
 
     _ = kw.get('extra_link_args', [])[:]
@@ -73,7 +78,8 @@ def _with_defaults(what, *argv, **kw):
     return what(*argv, **kw)
 
 
-def Ext(*argv, **kw):   return _with_defaults(_Ext, *argv, **kw)
+def PyGoExt(*argv, **kw):   return _with_defaults(_PyGoExt, *argv, **kw)
+def DSO(*argv, **kw):       return _with_defaults(_DSO, *argv, **kw)
 
 
 # build_py that
@@ -245,7 +251,7 @@ setup(
     keywords    = 'bigdata out-of-core numpy virtual-memory',
 
     ext_modules = [
-                    Ext('wendelin.bigfile._bigfile',
+                    PyGoExt('wendelin.bigfile._bigfile',
                         ['bigfile/_bigfile.c',
                          'bigfile/pagefault.c',
                          'bigfile/pagemap.c',
@@ -255,7 +261,8 @@ setup(
                          'bigfile/virtmem.c',
                          'lib/bug.c',
                          'lib/utils.c'],
-                        define_macros   = [('_GNU_SOURCE',None)]),
+                        define_macros   = [('_GNU_SOURCE',None)],
+                        language        = 'c'),
                   ],
 
     package_dir = {'wendelin': ''},
@@ -267,7 +274,7 @@ setup(
                    'ZODB >= 4', # for ZBigFile / ZBigArray
                    'zodbtools >= 0.0.0.dev8', # lib.zodb.dbstoropen + ...
 
-                   'pygolang >= 0.0.2', # defer, sync.WaitGroup, ...
+                   'pygolang >= 0.0.8', # defer, sync.WaitGroup, pyx/nogil channels ...
 
                    'six',       # compat py2/py3