More pre-2.3 build tweaks for the OS/2 EMX port:

More pre-2.3 build tweaks for the OS/2 EMX port:

- separate the building of the core from the wrapper executables and the
  external modules (.PYDs), based on the Py_BUILD_CORE define;

- clean up the generated import library definiton (.DEF file) to remove
  references to a number of non-static symbols that aren't part of the
  Python API and which shouldn't be exported by the core DLL;

- compile the release build with -fomit-frame-pointer, for a small
  performance gain;

- make "make clean" remove byte compiled Python library files.