macosx: fix handling of video-on-top (close #9615)
dbus inhibit: gnome >= 3.6 and < 3.10 is not supported

It uses the correct path name "/org/freedesktop/ScreenSaver" for the interface,
not the name that KDE came up with ("/ScreenSaver").
Gnome 3.10 supports the KDE interface now, see
https://mail.gnome.org/archives/commits-list/2013-September/msg00736.html

Support for gnome 3.6 screensaver is probably not worth it, as the daemon
crashes on Uninhibit.
Fix LUAC check if CONTRIB_DIR is not set