Georg, are you still able to reproduce this? I've subsequently tried on a Debian platform with an installed clang and could not reproduce.  And, after further inspecting the current code, I really don't see how the clang substitution could happen on a non-darwin platform.  My initial speculation about test_unixcompiler was incorrect as well as the proposed patch to preserve config vars in it (that test does not actually affect _config_vars and _config_vars is a dict so has to be deep-copied to be restorable).  I'm proceeding with the _osx_support consolidation but I sure would like to know what was behind the behavior you reported.