What is and how do you get the building on top of the rocks just north of Jacks hanger?

That landmark is the Aviary. You will receive it when you complete all the other landmarks: lighthouse, chapel, pirate ship, and mountain chateau. If you renovate them daily, expect about 2 years of playing before you receive the Aviary. The Aviary has 6 level, after which you can hatch birds and release them to fly around your island.