gtk+3 support has been added upstream, so add an option for that.
(From OE-Core rev: 0dc44aeb49a8a254ca3345870b62282b04d4b61f)
Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>