build: new --with-gstreamer-backend=[tagreadbin,decodebin2,discoverer]
build: new --with-gstreamer-backend=[tagreadbin,decodebin2,discoverer]

Allows selecting among 'tagreadbin', 'decodebin2' and 'discoverer', and replaces
the previous --enable-gstreamer-tagreadbin boolean switch.

Also makes sure that discoverer is requested when enabling the GUPnP-DLNA
enabled GStreamer extractor

outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker