meson: better error for x11+gl deps

Helps people avoid egl in the rare case they don't have it.

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

libweston/meson.build

@@ -364,7 +364,11 @@ if get_option('backend-x11')
 	endif
 
 	if get_option('renderer-gl')
-		deps_x11 += dependency('egl')
+		d = dependency('egl', required: false)
+		if not d.found()
+			error('x11-backend + gl-renderer requires egl which was not found. Or, you can use \'-Dbackend-x11=false\' or \'-Drenderer-gl=false\'.')
+		endif
+		deps_x11 += d
 	endif
 
 	plugin_x11 = shared_library(