Now that we require Meson 0.52 to build the doc, we can also pull
gtk-doc as subproject when missing from the system. This requires
to pull gtk-doc master because needed changes there haven't been release
yet.
Those lists were getting very long. We can’t quite entirely automate the
list generation since Meson doesn’t have a range() function, but we can
at least combine three of them into one.
Signed-off-by: Philip Withnall <withnall@endlessm.com>