Fixed: building on 10.6 failed.

There are features of Objective-C 2.0 used in XcodeCapp that cannot be compiled with Xcode 4.2, which is the maximum version that runs on OS X 10.6. Users on 10.6 would see compile errors.

Now a check for OS X 10.7+ is done, and on any version earlier than that, the user is presented with a message telling them to download XcodeCapp from the web site.