gnome_bg_set_draw_background removed from libgnome 3.8

On gentoo, compile fail with libgnome 3.8
undefined reference to `gnome_bg_set_draw_background'

background management changed :
https://git.gnome.org/browse/nautilus/commit/?h=gnome-3-8&id=372c60d3f8590280df23f457a67652adec369697
https://git.gnome.org/browse/nautilus/commit/?h=gnome-3-8&id=8b4a83ea80bf657cd8be754786e5f9daf7d5cc52

here is a minimal fix, but bigger work should be needed