audio/libcanberra: remove gtk2 dependency leftover

Gtk2 dependency was leftover after the split of libcanberra-gtk[1]
while not used, remove it.

While here explicitly disable udev (which can get activated if
libudev-devd is installed on the building machine).

Also drop the gtkdoc entry from the default Makefile as the
--disable-gtk-doc switch it not working.

Reported by: tijl