Relax Gtk+ requirement slightly
    
    We previously bumped the gtk+ requirement to 3.18 for the function
    gtk_window_fullscreen_on_monitor(). But this function is only necessary
    in Wayland. So add some preprocessor version checks to allow it to
    compile on older distributions if they don't care about wayland support.
    
    Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>