Adding a new feature to help support building for the Windows Runtime (Windows store/phone).
1. Adds a new feature <windows-api> for specifying the WINAPI_FAMILY API set to target. Defaults to desktop, which is how builds before my changes.
2. A few small changes for adding ARM support.
3. Changes to call to correct vcvars setup script for Windows Phone 8.
Removing use of testing.execute feature until it gets added in separate pull request.
Updating to include support for Windows Phone 8.1.
Addressing feedback from review:
1. Fixing up a bit how the phone setup script location is calculated.
2. Reusing generate-setup-cmd rule in configure-really to avoid duplication and make cleaner.
3. Adding some more comments explaining.
Support building for the Windows Runtime.

This merges https://github.com/boostorg/build/pull/14.