scons: Add X11 include path if X11 is available.

Mac OS X XQuartz places X11 headers at /opt/X11/include.

This patch fixes this Mac OS X SCons build error.

  Compiling src/gallium/state_trackers/glx/xlib/glx_api.c ...
In file included from src/gallium/state_trackers/glx/xlib/glx_api.c:34:
include/GL/glx.h:30:10: fatal error: 'X11/Xlib.h' file not found
         ^

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
1 file changed