Here's a simple test-case that mimics what xulrunner does.

=============================
#include <X11/extensions/render.h>

int main (int argc, char** argv)
{
        return 0;
}
=============================

Here's the build result :

$ gcc render.c -o render-test `pkg-config --libs --cflags xrender`
In file included from render.c:1:
/usr/include/X11/extensions/render.h:29: erreur: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘Glyph’
/usr/include/X11/extensions/render.h:30: erreur: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘GlyphSet’
/usr/include/X11/extensions/render.h:31: erreur: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘Picture’
/usr/include/X11/extensions/render.h:32: erreur: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘PictFormat’

Adding a #include <X11/X.h> fixes the build issue (for the test case and xulrunner), but I haven't tried building anything else (mainly the server).

FTR, here's the Gentoo bug report : http://bugs.gentoo.org/238665

Thanks