Re-enable Xcode-only support on 10.9

tl;dr we can safely re-enable Xcode-only support on 10.9.

I've spent some time over the past few weeks experimenting with a VM running 10.9 with only Xcode installed.

Along the way I've discovered and fixed a number of bugs in superenv, as well as in individual packages, that prevented things from working well on Xcode-only systems going back to 10.7.

While I think early Xcode 5 builds probably had a few bugs, a lot of problems we diagnosed as bugs in Xcode were really caused by a combination of assumptions of previous versions that do not apply on 10.9, and failure to adapt to changes in how Xcode is packaged.