meson: better errors for VA-API deps

Helps people avoid libva if they don't want it.

Signed-off-by: Pekka Paalanen <pq@iki.fi>

libweston/meson.build

@@ -192,12 +192,16 @@ if get_option('backend-drm')
 	endif
 
 	if get_option('backend-drm-screencast-vaapi')
+		foreach name : [ 'libva', 'libva-drm' ]
+			d = dependency(name, version: '>= 0.34.0', required: false)
+			if not d.found()
+				error('VA-API recorder requires @0@ >= 0.34.0 which was not found. Or, you can use \'-Dbackend-drm-screencast-vaapi=false\'.'.format(name))
+			endif
+			deps_drm += d
+		endforeach
+
 		srcs_drm += 'vaapi-recorder.c'
-		deps_drm += [
-			dependency('libva', version: '>= 0.34.0'),
-			dependency('libva-drm', version: '>= 0.34.0'),
-			dependency('threads'),
-		]
+		deps_drm += dependency('threads')
 		config_h.set('BUILD_VAAPI_RECORDER', '1')
 	endif