scons: don't compile some files with -gstabs if using mingw32

scons: don't compile some files with -gstabs if using mingw32

Compiling some (large) files with i686-pc-mingw32-gcc 4.2.2 (at least)
and the -gstabs option triggers a compiler error.  Use this work-around
to simply compile the effected files without -gstabs.