autotools: make libtool override optional
autotools: make libtool override optional

nirbheek/cerbero,atsushieno/cerbero,atsushieno/cerbero,atsushieno/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,atsushieno/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,atsushieno/cerbero