Draft: build: Add a subproject for wayland

It allows building mutter even if the installed wayland-server / client is older than the required one

The first commit could be considered a hack around the limitation, but gtk/glib/gdk-pixbuf/pango seems to be doing that as well.

After asking around in #hackers it seems to not be a "hack" actually, but rather what should libraries do.

Marked as a draft until I figure out how to make meson pull the requires information from the library object itself.