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