Fixes#2952. The testsuite indirectly called strftime() with conversion
specifiers defined only in C99. In MSVCRT this function conforms only to
C89. Only in the updated UCRT this functon provides the functionality of
C99.
Also replaces the SFIANE tricks applied in
ce7ecdb7af with conditional compilation.
The code was too complicated along with the other trick to access private
data members.
* Refactor detail::print() by splitting into two functions.
The part with SetConsoleW is a separate function, without fwrite().
* Make Unicode handing when writing to std::ostream more robust.
Calls to print(ostream&) in the special Unicode case on Windows fallback
to writing via ostream::write instead of fwrite().
* Fix Unicode handling when writing to an ostream on GCC on Windows
* Add TODO note about detail::is_utf8()
* Fix warning -Wundef
* Fix for non-Windows OSs
* Fix building as DLL on Windows
* Refactor
* Suppress warning
Dimitrij Mijoski