Fix compilation with lua 5.2 and 5.3

Mostly to have the lua53 "library" work with both lua 5.1 and lua 5.3.
Additionally fixes two other incompatibilities.

--HG--
branch : minor

src/common/runtime.cpp

@@ -135,7 +135,7 @@ int luax_traceback(lua_State *L)
 	if (!lua_isstring(L, 1))  // 'message' not a string?
 		return 1; // keep it intact
 
-	lua_getfield(L, LUA_GLOBALSINDEX, "debug");
+	lua_getglobal(L, "debug");
 	if (!lua_istable(L, -1))
 	{
 		lua_pop(L, 1);