gtk+3: port to Meson

GTK+ 3.24.36 will drop the autotools build, so get ahead of the curve
and switch to Meson.

The tarball is missing one meson.build file so add that explictly for
now.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>