SDL 1.2 unconditionally defines _GNU_SOURCE via it's sdl-config script.
The new version of port games/0ad(which I'm testing) fails to build with such a a define. It's also anyway wrong to force such a define on anything depending on sdl 1.2.
The attached patch I'm proposing for review is taken from upstream:
http://hg.libsdl.org/SDL/rev/7b87c1773b3e
and removes such define.
I was pointed to it by one of the 0ad developers.