#ifdef

I was wondering which define we should test in ported code. For now I used __APPLE__ but this brakes the code for X11 compilation. I saw that the Windows Port defines G_OS_WIN32. Is there or will there be anything for the Mac OS X native port?

Easy.

GTK+ defines GDK_WINDOWING_*

GTK+ defines GDK_WINDOWING_* (for X11, WIN32 and QUARTZ).