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

Index: common/lualib.h
--- common/lualib.h.orig
+++ common/lualib.h
@@ -28,7 +28,7 @@
 #include "common/util.h"
 
 /** Lua function to call on dofunction() error */
-lua_CFunction lualib_dofunction_on_error;
+extern lua_CFunction lualib_dofunction_on_error;
 
 void luaA_checkfunction(lua_State *, int);
 void luaA_checktable(lua_State *, int);