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

Index: objects/button.h
--- objects/button.h.orig
+++ objects/button.h
@@ -39,7 +39,7 @@ typedef struct button_t
     xcb_button_t button;
 } button_t;
 
-lua_class_t button_class;
+extern lua_class_t button_class;
 LUA_OBJECT_FUNCS(button_class, button_t, button)
 ARRAY_FUNCS(button_t *, button, DO_NOTHING)