Recently I mentioned PSTCollectionView a project that brings a UICollectionView compatible component to iOS 4.3 to help maintain compatibility with those running older iOS versions.
Rolandas Razma has done the same with the excellent auto layout feature of iOS 6.
There have been some other open source auto layout libraries in the past, but this library, RRAutoLayout, is designed to maintain compatibility with the iOS 6 auto layout API and works with iOS 5.
You can find RRAutoLayout on Github here.
Roland’s also written an extensive writeup on the Github page explaining how to use the component, and has included a couple of example projects.