50.0.0 can't build extensions for Python < 3.8 if Py_ENABLE_SHARED

[godlygeek]

For Python 3.7 and lower, if the interpreter is built as Py_ENABLE_SHARED, the interpreter core is built into a libpython shared library, and extension modules built for that interpreter need to link against that shared library when they are built. This changed in 3.8:

On Unix, C extensions are no longer linked to libpython except on Android and Cygwin.

setuptools 50.0.0 does not include libpython in the link command when building for a Python 3.7 Py_ENABLE_SHARED interpreter, so extensions that can build successfully with 49.6.0 fail to link with 50.0.0.

Presumably this is because 50.0.0 is using a vendored distutils that was copied from a recent CPython, and support for linking against libpython was dropped there because it is no longer used after 3.8, but it is still required for older versions.

[godlygeek]

Specifically python/cpython@8c3ecc6 removed the logic to use Py_ENABLE_SHARED to decide whether to include libpython in the link command, and it was later added back in for Cygwin and Android. The version vendored into 50.0.0: https://github.com/pypa/setuptools/blob/17cb9d6bf249cefe653d3bdb712582409035a7db/setuptools/_distutils/command/build_ext.py#L736-L750 never sets link_libpython to True on non-Cygwin, non-Android platforms.

[jaraco]

Fix releasing with setuptools 50.0.3.