the platform-neutral include dir by default and with Mac support.
Added 'get_python_lib()', inspired by 'get_python_inc()'.
Rewrote 'get_config_h_filename()' and 'get_makefile_filename()'
  in terms of 'get_python_inc()' and 'get_python_lib()'.
Changed '_init_nt()' and '_init_mac()' to use 'get_python_inc()' and
  'get_python_lib()' for directory names.