x11: Query whether we have GLX support

Epoxy 1.4 has new ad hoc API that we can use to check whether GLX is
available on the current system.

If we didn't use this API, we'd have to manually dlopen libGL (or its
equivalent on different OSes) and check if it had GLX symbols; since
Epoxy already does all of this internally, we can simply ask it instead.

https://bugzilla.gnome.org/show_bug.cgi?id=775279

gdk/x11/gdkglcontext-x11.c

@@ -799,6 +799,9 @@ gdk_x11_screen_init_gl (GdkScreen *screen)
 
   dpy = gdk_x11_display_get_xdisplay (display);
 
+  if (!epoxy_has_glx (dpy))
+    return FALSE;
+
   if (!glXQueryExtension (dpy, &error_base, &event_base))
     return FALSE;