freebox: Check for avahi-glib and avahi-client
freebox: Check for avahi-glib and avahi-client

freebox plugin links against libavahi-client and libavahi-glib,
in addition to libavahi-gobject.

https://bugzilla.gnome.org/show_bug.cgi?id=724369

MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,grilofw/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins