Automatically select the wayland backend if WAYLAND_SOCKET is set

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/compositor.c b/src/compositor.c
index 6b90bd8..362e959 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -4168,7 +4168,7 @@
 						 NULL);
 
 	if (!backend) {
-		if (getenv("WAYLAND_DISPLAY"))
+		if (getenv("WAYLAND_DISPLAY") || getenv("WAYLAND_SOCKET"))
 			backend = strdup("wayland-backend.so");
 		else if (getenv("DISPLAY"))
 			backend = strdup("x11-backend.so");