`MacOS::Xcode.version > "4.1"`

There are a few different cases we could check for:

* Reported Xcode version is greater than 4.1, but Xcode 4.1's compilers are installed (e.g. clang 2.1 / build 163)
* Mountain Lion or greater has a reported Xcode version of 4.1

Unfortunately it's much harder to detect osx-gcc-installer on Snow Leopard or Lion as there actually was an Xcode 4.1 release on those platforms, but it's also less painful there. Probably we want to check for either of those two cases above.