Prepare for making MetaX11Display conditional

- Stop using CurrentTime, introduce META_CURRENT_TIME
- Use g_get_monotonic_time () instead of relying on an
  X server running and making roundtrip to it

https://bugzilla.gnome.org/show_bug.cgi?id=759538

src/core/restart.c

@@ -195,7 +195,7 @@ meta_restart_finish (void)
       MetaDisplay *display = meta_get_display ();
       Display *xdisplay = meta_x11_display_get_xdisplay (display->x11_display);
       Atom atom_restart_helper = XInternAtom (xdisplay, "_MUTTER_RESTART_HELPER", False);
-      XSetSelectionOwner (xdisplay, atom_restart_helper, None, CurrentTime);
+      XSetSelectionOwner (xdisplay, atom_restart_helper, None, META_CURRENT_TIME);
     }
 }