ccompiler: restore customize_compiler() setup for macOS. See #268
The original customize_compiler() calls into _osx_support.customize_compiler() the first time it is used and I didn't copy it in !118 because it is private API. Issue #268 points out that the macOS build is broken now so I guess that was important in some way. Make sure the setup code is run by calling the original customize_compiler() with a dummy compiler instance.