Fix supported Xcode versions for bootstrap using Xcode 5 on 10.8

This fixes the supported Xcode versions so that folks can use Xcode 5.  The following commit from Futureproof2 is related (actually included in this request):

https://github.com/fink/fink/commit/06892ff110133f861f2f57c648ef9de2469f00ae#commitcomment-4137034