Keep NDEBUG and DEBUG_libfirm in sync in the Makefile.
Keep NDEBUG and DEBUG_libfirm in sync in the Makefile.

This unbreaks the build with !NDEBUG but DEBUG_libfirm.

MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm