Fix nearly all compilation warnings under Apple gcc-4.0. Tested with OPT="-g

-Wall -Wstrict-prototypes -Werror" in both --with-pydebug mode and --without.
There's still a batch of non-prototype warnings in Xlib.h that I don't know how
to fix.

Modules/_struct.c

@@ -182,6 +182,7 @@ get_long(PyObject *v, long *p)
 
 /* Same, but handling unsigned long */
 
+#ifndef PY_STRUCT_OVERFLOW_MASKING
 static int
 get_ulong(PyObject *v, unsigned long *p)
 {
@@ -201,6 +202,7 @@ get_ulong(PyObject *v, unsigned long *p)
 	}
 	return 0;
 }
+#endif  /* PY_STRUCT_OVERFLOW_MASKING */
 
 #ifdef HAVE_LONG_LONG