MSVC: Add flags for C++ language standards
Visual Studio 2015 Update 3 introduced the notion of language standard
levels to MSVC. The language standard level is defined in _MSVC_LANG
instead of __cplusplus. It also added support for the -std:c++14
and -std:c++latest flags, although the compiler defaults to its C++14
mode anyway. Visual Studio 2017 Update 3 will introduce support for the
-std:c++17 flag.
Fixes: #16482 (closed)