meson: better errors for gl-renderer egl and glesv2

Helps people to avoid EGL and GLESv2 if they do not want them.

Stops using dep_egl and dep_glesv2 so that the human friendly error
message is alongside the dependency() statement, so that the message and
the statement can later be merged together once Meson offers the custom
error messages feature or something even more sophisticated.

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

libweston/meson.build

@@ -413,10 +413,6 @@ endif
 if get_option('renderer-gl')
 	config_h.set('ENABLE_EGL', '1')
 
-	if not dep_egl.found() or not dep_glesv2.found()
-		error('gl-renderer enabled but EGL/GLESv2 not found')
-	endif
-
 	srcs_renderer_gl = [
 		'gl-renderer.c',
 		'vertex-clipping.c',
@@ -424,13 +420,21 @@ if get_option('renderer-gl')
 		linux_dmabuf_unstable_v1_protocol_c,
 		linux_dmabuf_unstable_v1_server_protocol_h,
 	]
+
 	deps_renderer_gl = [
-		dep_egl,
-		dep_glesv2,
 		dep_pixman,
 		dep_libweston,
 		dep_libdrm_headers,
 	]
+
+	foreach name : [ 'egl', 'glesv2' ]
+		d = dependency(name, required: false)
+		if not d.found()
+			error('gl-renderer requires @0@ which was not found. Or, you can use \'-Drenderer-gl=false\'.'.format(name))
+		endif
+		deps_renderer_gl += d
+	endforeach
+
 	plugin_gl = shared_library(
 		'gl-renderer',
 		srcs_renderer_gl,