Mac install fix

On *Mac* `qlua` was compiled with `lua` instead of `luajit`.
Now on **both** `Linux` and `Darwin` (I've tested them on *Ubuntu* 13.10 and *OSX* 10.9.2) `qlua` is compiled correctly with `luajit`, therefore allowing `qlua` to use `ffi` which is necessary, for example, for the `image` package to work properly.