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>
3 jobs for meson-override-deps in 3 minutes and 53 seconds (queued for 3 seconds)
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debian:buster@container-prep

00:02:31

 
  Build
passed #6463436
build-native-autotools

00:01:22

passed #6463437
build-native-meson

00:00:37