X11_CFLAGS is never defined. Path to X11 headers is not needed here, so
just remove.
Future work: Using AM_CFLAGS here looks wrong, as this Makefile only builds
C++ files
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk> Reviewed-by: Matt Turner <mattst88@gmail.com> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>