make libcanberra support optional
make libcanberra support optional

Make it possible to control libcanberra support so we don't risk ending up with
a cyclic dependency when using packages: gtk+ -> libcanberra-gtk3 -> gtk+

https://bugzilla.gnome.org/show_bug.cgi?id=746904

ahodesuka/gtk,jigpu/gtk,grubersjoe/adwaita,chergert/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,grubersjoe/adwaita,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,ahodesuka/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,davidgumberg/gtk,ahodesuka/gtk,grubersjoe/adwaita,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,jadahl/gtk,chergert/gtk,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,ahodesuka/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,Adamovskiy/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,davidgumberg/gtk,chergert/gtk