Versioned Clang automatic configuration

[Kojoley]

Currently, without annoying toolset.using directives in user-config.jam, b2 toolset=clang-xx silently uses clang++ binary, even if it is a different version than requested. Instead of copycating GCC or reinventing a wheel I have generalized GCC automatic configuration and used it for Clang.

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[Kojoley]

@grafikrobot is there anything stopping the PR from being merged?