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

Index: luaa.h
--- luaa.h.orig
+++ luaa.h
@@ -311,7 +311,7 @@ const char *luaA_find_config(xdgHandle *, const char *
 bool luaA_parserc(xdgHandle *, const char *);
 
 /** Global signals */
-signal_array_t global_signals;
+extern signal_array_t global_signals;
 
 int luaA_class_index_miss_property(lua_State *, lua_object_t *);
 int luaA_class_newindex_miss_property(lua_State *, lua_object_t *);