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

Index: objects/window.h
--- objects/window.h.orig
+++ objects/window.h
@@ -80,7 +80,7 @@ typedef struct
     WINDOW_OBJECT_HEADER
 } window_t;
 
-lua_class_t window_class;
+extern lua_class_t window_class;
 
 void window_class_setup(lua_State *);