configure.ac: GLX cannot work without OpenGL

configure.ac: GLX cannot work without OpenGL

GLX uses mapi/glapi/libglapi.la, which is only built for OpenGL.

If the user specified --enable-xlib-glx --disable-opengl, error out, as these
cannot be both observed at the same time. If the user just specified
--disable-opengl but not --disable-glx, print a warning and disable GLX as
well.

NOTE: This is a candidate for the stable branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59364

Tested-by: Tom Stellard <thomas.stellard@amd.com>