Consolidate detection of C++17 features like std::chrono::floor

[ja11sop]

Commit 59d9cfa breaks gcc 6+ and clang 3.8+ when --std=c++1z is set because the _cplusplus macro is defined as 201500 and 201406 respectively. In other words not whatever value it will be to indicate C++17 support. That libc++ (I haven't checked) might already offer some of those features should be detected for separately, so the work-arounds in date.h are still provided for gcc and clang that build against libstdc++. (running Linux).

[HowardHinnant]

Thanks! Please reopen this issue if this is not addressed by 845ce25