Attached is a diff (04610583238f.diff) containing a second revision of my implementation of this feature. It incorporates suggestions made during code review, as well as adds unittests and documentation of the new features.

This patch retains the same basic behavior as the previous patch, with only a few small changes:

* packaging.config.Config now uses ";;" as the environment marker separator for specific_compile_args and specific_link_args (to match the patch I submitted under issue 12424). 

* BCPPCompiler.get_compiler_version() has been implemented. This was the only compiler lacking an implementation under my previous patch.

* UnixCCompiler.get_compiler_version() has been enhanced to parse more version string formats, to better handle unknown compilers.

Outside of that, this patch adds unittests for all the new functionality; as well as documents CCompiler.get_compiler_version(), the new keywords added to Extension, how they both affect build_ext.

This should cover all the previously identified issues, let me know if there's anything more I can do to improve the patch.