On some Windows compilers, the constant PATH_MAX may already be defined, which will cause compile errors on non-MSVC compilers (notably Open Watcom and MinGW).  Rather than assume it is not available and define it in all "#ifdef MS_WINDOWS" cases, it should first be checked for existence.

This patch adds said check to Modules/main.c and Python/pythonrun.c.  This patch should not affect any existing platforms (including MSVC builds).