meson: better errors for x11-backend deps

Helps people avoid X11 related dependencies if they don't want them.

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

libweston/meson.build

@@ -334,15 +334,25 @@ if get_option('backend-x11')
 		presentation_time_server_protocol_h,
 	]
 
+	dep_x11_xcb = dependency('xcb', version: '>= 1.8', required: false)
+	if not dep_x11_xcb.found()
+		error('x11-backend requires xcb >= 1.8 which was not found. Or, you can use \'-Dbackend-x11=false\'.')
+	endif
+
 	deps_x11 = [
-		dependency('xcb', version: '>= 1.8'),
+		dep_x11_xcb,
-		dependency('xcb-shm'),
-		dependency('x11'),
-		dependency('x11-xcb'),
 		dep_lib_cairo_shared,
 		dep_pixman,
 	]
 
+	foreach name : [ 'xcb-shm', 'x11', 'x11-xcb' ]
+		d = dependency(name, required: false)
+		if not d.found()
+			error('x11-backend requires @0@ which was not found. Or, you can use \'-Dbackend-x11=false\'.'.format(name))
+		endif
+		deps_x11 += d
+	endforeach
+
 	dep_xcb_xkb = dependency('xcb-xkb', version: '>= 1.9', required: false)
 	if dep_xcb_xkb.found()
 		deps_x11 += dep_xcb_xkb