Change the gacdir from $(libdir) to $(prefix)/lib
Change the gacdir from $(libdir) to $(prefix)/lib

On SUSE* distros at least, the Mono GAC is at /usr/lib/mono even on 64 bit
installs where libdir is /usr/lib64.

Fixes bug #606680.

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