[Python-Dev] Defining _XOPEN_SOURCE in Python.h breaks 2.0b1 on Tru64 Unix

[Favas, Mark (EM, Floreat)]

This change results in mathmodule.c failing to compile (rint is undefined)
due to an interaction between defining _XOPEN_SOURCE and the use of
_XOPEN_SOURCE_EXTENDED in the system includes (/usr/include/math.h). Yes,
configure correctly defines HAVE_RINT. This change also results in a number
of errors and warnings in timemodule.c, and probably further modules down
the compilation chain.