Fix -fno-common build failure. Taken from
https://github.com/awesomeWM/awesome/commit/d256d9055095f27a33696e0aeda4ee20ed4fb1a0

Index: objects/client.h
--- objects/client.h.orig
+++ objects/client.h
@@ -192,7 +192,7 @@ struct client_t
 ARRAY_FUNCS(client_t *, client, DO_NOTHING)
 
 /** Client class */
-lua_class_t client_class;
+extern lua_class_t client_class;
 
 LUA_OBJECT_FUNCS(client_class, client_t, client)