meson: override dependencies to ease use as subproject

Make it easier to use Wayland as a Meson subproject by overriding
dependencies we define. This allows to easily build Wayland as a
subproject like so:

    subproject('wayland', required: false, default_options: ['documentation=false'])

After this statement, the wayland-* dependencies will use the subproject
instead of the system if available.

Signed-off-by: Simon Ser <contact@emersion.fr>