meson: better errors for xwayland deps

Helps people to disable xwayland if they don't want to install the
dependencies.

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

xwayland/meson.build

@@ -9,15 +9,26 @@ srcs_xwayland = [
 	'dnd.c',
 	'hash.c',
 ]
-deps_xwayland = [
+
-	dependency('xcb'),
+dep_names_xwayland = [
-	dependency('xcb-composite'),
+	'xcb',
-	dependency('xcb-shape'),
+	'xcb-composite',
-	dependency('xcb-xfixes'),
+	'xcb-shape',
-	dependency('xcursor'),
+	'xcb-xfixes',
-	dependency('cairo-xcb'),
+	'xcursor',
-	dep_libweston,
+	'cairo-xcb',
 ]
+
+deps_xwayland = [ dep_libweston ]
+
+foreach name : dep_names_xwayland
+	d = dependency(name, required: false)
+	if not d.found()
+		error('xwayland requires @0@ which was not found. Or, you can use \'-Dxwayland=false\'.'.format(name))
+	endif
+	deps_xwayland += d
+endforeach
+
 plugin_xwayland = shared_library(
 	'xwayland',
 	srcs_xwayland,