build: substitute X11_INCLUDES variable

There are a few automake files that reference $(X11_INCLUDES) such as
src/glx/Makefile.am but configure.ac wasn't declaring the variable for
substitution. This would break builds of glx if libxcb, for example, was
installed in its own prefix since AM_CFLAGS wouldn't coincidentally
list the needed include path in that case.

Reviewed-by: Matt Turner <mattst88@gmail.com>
1 file changed