media-libs/libtxc_dxtn lacks a dep on mesa. Trying to emerge it without mesa installed will fail since GL/gl.h is not there.
Looks like you're right. I guess I've never noticed because I always emerged it after mesa once I realized I was missing some GL extensions. :) I'll fix this when I get back to my computer with my cvs tree unless someone beats me to it.
Fixed in cvs. :-)