Add C++14 support based on https://github.com/boostorg/config/pull/39
Fix spelling of one C++14 macro, improve C++14 auto-detection.
Fix broken <atomic> with clang, tentatively fix clang number separator option.
Improve GCC C++14 feature detection.
Merge branch 'cxx14' into develop
Fix C++14 macro docs.  Regen docs.