Wider Lua compatibility...

[snichols]

Simplifying code that deals with various versions of Lua.

[ghost]

LuaJit 2.1

#if LUA_VERSION_NUM < 502
static void luaL_setmetatable(lua_State *L, const char *tname) {
    luaL_getmetatable(L, tname);
    lua_setmetatable(L, -2);
} /* luaL_setmetatable() */
#endif