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

Index: common/lualib.c
--- common/lualib.c.orig
+++ common/lualib.c
@@ -20,6 +20,8 @@
 #include "common/lualib.h"
 #include "luaa.h"
 
+lua_CFunction lualib_dofunction_on_error;
+
 void luaA_checkfunction(lua_State *L, int idx)
 {
     if(!lua_isfunction(L, idx))