Fix clang-win.jam so that it works

[pdimov]

No description provided.

[pdimov]

Fixes #330.

[pdimov]

With this in place, clang-cl should now be usable on Appveyor (which has it installed and in PATH), by just using toolset=clang-win.

[eldiener]

The new clang-win.jam worked well. Is there a way to choose a VC++ version as the backend headers and libraries, or is the latest VC++ version found always chosen ? The old clang-win had a property but the new one does not have anything like that which I can see.

[pdimov]

I'm not sure how clang-cl picks the version to use. Will need to check the sources. I think that if you invoke it from a developer command prompt, it uses whatever is set by it, but if not, it scans and picks the most recent MSVC toolset (and Windows SDK - these two are separate.)

[github-actions]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2 Please consider following up at https://github.com/bfgroup/b2/pulls