It's hard to sort out what the bug was, exactly.  So, Big Hammer:

1. Python shouldn't be in the business of #define'ing NULL, period.
2. Users of the Python C API shouldn't be in the business of not including
   Python.h, period.

Hence:

1. Removed all #define's of NULL in Python source code (pyport.h and
   object.h).
2. Since we're *relying* on stdio.h defining NULL, put an #error in
   Python.h after its #include of stdio.h if NULL isn't defined then.