Fixing build break with Visual Studio if phone scripts aren't installed with VS 2012/2013

Fixes a late issue discovered that can cause build issues if Visual Studio 2012 or 2013 doesn't actually contain the Windows Phone setup scripts.

This was caused from pull request https://github.com/boostorg/build/pull/14. I've tested now on a machine without the phone setup scripts for Visual Studio 2012 and 2013.