build: avoid Xlib.h via EGL headers
This should make EGL headers not pull in Xlib headers when no specific platform define is in effect.
Use both the old Mesa-specific hack and the new official define EGL_NO_X11 to have this work on both old and new EGL headers.
Fixes: #350 (closed)
Signed-off-by: Pekka Paalanen pekka.paalanen@collabora.com