Go back to using WAF's check_python_headers on non-Windows platforms.
It is needed in case the Python installation has split the pyconfig.h to
a different location, so the extra -I flag will be added.

git-svn-id: http://svn.wxwidgets.org/svn/wx/wxPython@73662 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775