build: use meson.override_dependency

When built as a subproject, this removes the need for the parent project
to know about the dependency variable name.

This requires Meson 0.54.0.

meson.build

@@ -3,7 +3,7 @@ project(
 	'c',
 	version: '0.10.0',
 	license: 'MIT',
-	meson_version: '>=0.51.2',
+	meson_version: '>=0.54.0',
 	default_options: [
 		'c_std=c11',
 		'warning_level=2',
@@ -167,6 +167,8 @@ wlroots = declare_dependency(
 	include_directories: wlr_inc,
 )
 
+meson.override_dependency('wlroots', wlroots)
+
 summary = [
 	'',
 	'----------------',