A cross-compile of a Win32 Python under Linux using
MingW (www.mingw.org) didn't work, as posixmodule.c
didn't know anything about MingW.
This patch fixes the various compile errors by
introducing a new local MINGW define in posixmodule.c,
and making several #ifdefs also take that variable into
account.

After this patch is applied, it's possible to compile
python.exe on Linux with MingW by doing

 CXX=i386-mingw32msvc-g++ \
 CPP=i386-mingw32msvc-cpp \
 RANLIB=i386-mingw32msvc-ranlib \
 AR=i386-mingw32msvc-ar \
 CC="/opt/xmingw/bin/i386-mingw32msvc-gcc -DMS_WINDOWS
-DPy_WIN_WIDE_FILENAMES" \
 ./configure --host=i586-mingw32msvc 

 sed -i s/initposix/initnt/g Modules/config.c
 sed -i 's/#define HAVE_DEV_PTMX 1//g' pyconfig.h
   
 make