build: declare dependency for use as a subproject

This allows clients and compositors to easily use wayland-protocols
as a Meson subproject.

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

meson.build

@@ -1,6 +1,6 @@
 project('wayland-protocols',
 	version: '1.21',
-	meson_version: '>= 0.53.0',
+	meson_version: '>= 0.54.0',
 	license: 'MIT/Expat',
 )
 
@@ -107,6 +107,14 @@ configure_file(
 	configuration: pkgconfig_configuration,
 )
 
+wayland_protocols = declare_dependency(
+	variables: {
+		'pkgdatadir': wayland_protocols_srcdir,
+	},
+)
+
+meson.override_dependency('wayland-protocols', wayland_protocols)
+
 if get_option('tests')
 	subdir('tests')
 endif