Add support for `stdlib=libc++` for clang?

[pdimov]

The feature already exists, was used for stlport which seems now obsolete, but seems the right fit for clang++ -stdlib=libc++.

[stale[bot]]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2 This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.