Xcode 4.3.1 changes

[xgalaxy]

Don't pull this in until you've moved over to the new Xcode.

This pull request has the following changes:

1. Fixes an issue where minimum sdk version (aka deployment version) was not being properly set.
2. Fixes an issue where compiler / linker min sdk version switch was not being properly set.
3. Now supports Xcode 4.3.1. These changes are not backwards compatible (though a bit more work and they possibly could be. Xcode 4.3.1 moves the locations of various tools, etc.