[build] Make GIO optional with --disable-gio configure switch.
[build] Make GIO optional with --disable-gio configure switch.

Fixes bgo#621315.

stsundermann/banshee,stsundermann/banshee,GNOME/banshee,stsundermann/banshee,mono-soc-2011/banshee,lamalex/Banshee,GNOME/banshee,mono-soc-2011/banshee,Carbenium/banshee,dufoli/banshee,babycaseny/banshee,mono-soc-2011/banshee,ixfalia/banshee,directhex/banshee-hacks,Carbenium/banshee,ixfalia/banshee,allquixotic/banshee-gst-sharp-work,dufoli/banshee,allquixotic/banshee-gst-sharp-work,mono-soc-2011/banshee,mono-soc-2011/banshee,stsundermann/banshee,allquixotic/banshee-gst-sharp-work,lamalex/Banshee,Dynalon/banshee-osx,dufoli/banshee,arfbtwn/banshee,allquixotic/banshee-gst-sharp-work,GNOME/banshee,ixfalia/banshee,directhex/banshee-hacks,stsundermann/banshee,Carbenium/banshee,Dynalon/banshee-osx,lamalex/Banshee,babycaseny/banshee,Carbenium/banshee,mono-soc-2011/banshee,ixfalia/banshee,babycaseny/banshee,babycaseny/banshee,arfbtwn/banshee,Dynalon/banshee-osx,Dynalon/banshee-osx,dufoli/banshee,dufoli/banshee,arfbtwn/banshee,directhex/banshee-hacks,dufoli/banshee,babycaseny/banshee,babycaseny/banshee,Dynalon/banshee-osx,lamalex/Banshee,GNOME/banshee,dufoli/banshee,arfbtwn/banshee,directhex/banshee-hacks,arfbtwn/banshee,petejohanson/banshee,petejohanson/banshee,Carbenium/banshee,arfbtwn/banshee,ixfalia/banshee,petejohanson/banshee,stsundermann/banshee,GNOME/banshee,stsundermann/banshee,petejohanson/banshee,lamalex/Banshee,babycaseny/banshee,directhex/banshee-hacks,ixfalia/banshee,petejohanson/banshee,arfbtwn/banshee,GNOME/banshee,allquixotic/banshee-gst-sharp-work,babycaseny/banshee,Dynalon/banshee-osx,petejohanson/banshee,directhex/banshee-hacks,arfbtwn/banshee,GNOME/banshee,stsundermann/banshee,ixfalia/banshee,Dynalon/banshee-osx,lamalex/Banshee,dufoli/banshee,GNOME/banshee,allquixotic/banshee-gst-sharp-work,mono-soc-2011/banshee,ixfalia/banshee,Dynalon/banshee-osx,Carbenium/banshee