Configure: Use mcs instead of gmcs
Configure: Use mcs instead of gmcs

On some machines gmcs links to mscorlib 2.0 but that's too old, so we use the recommended mcs compiler.

freedesktop-unofficial-mirror/gstreamer__gstreamer-sharp,GStreamer/gstreamer-sharp,freedesktop-unofficial-mirror/gstreamer__gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,GStreamer/gstreamer-sharp,freedesktop-unofficial-mirror/gstreamer__gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,GStreamer/gstreamer-sharp