[build_ext] bulding shared libraries: sysconfig._CONFIG_VARS vs distutils.sysconfig._config_vars

Hello,

I was browsing the setuptools sources, and I discovered that it has (experimental?) support for building native shared libraries (.dylib, .dll, etc.) to be linked with Python extension modules.
I'm referring to setuptools.extension.Library class and the setup_shlib_compiler method of build_ext command.

I also noticed there was a tests/shlib_test (which somehow was removed with commit https://github.com/pypa/setuptools/commit/654c26f78a3031c34b81f338d3b6d1a3a87c34da). I only had to modify the "shlib_test/setup.py" script a bit to make it run with the current setuptools (21.0):

-from setuptools import setup, Extension, Library
+from setuptools import setup, Extension
+from setuptools.extension import Library

I encountered an issue in the setup_shlib_compiler method of build_ext command, where the compiler is configured for building shared libraries. When the platform is OSX ("darwin"), the global _CONFIG_VARS are temporarily modified (the flags -dynamiclib -undefined dynamic_lookup are required for .dylib); then the distutils.sysconfig.customize_compiler function is called in order to set these flags.
The problem is the distutils' customize_compiler function is reading from a different dictionary than then one setuptools is modifying, namely distutils.sysconfig._config_vars; therefore, the default values are used instead of the temporarily modified ones, and a -bundle (which can't be linked at compile time) instead of a -dynamiclib gets created.

The issue was introduced with this old commit https://github.com/pypa/setuptools/commit/c04abca6 which replaced all uses of distutils.sysconfig in setuptools, including build_ext module, with equivalent imports from the new sysconfig module, available since Python >= 2.7/3.2.

The issue can be fixed by importing _config_vars from distutils.sysconfig instead of the top-level sysconfig. Then the customize_compiler function can use the temporarily overridden variables as originally intended.

I hope this makes sense... :)

Start with the build_ext.setup_shlib_compiler method and compare its sysconfig._CONFIG_VARS import with distutils.sysconfig._config_vars and customize_compiler. Restore or adapt the tests/shlib_test setup.py entry point described in the issue, then verify that the OSX shared-library build uses the required dynamic-library flags rather than bundle flags.