If CXX_STANDARD is 17, then /std:c++17 should be passed to MSVC 14.11+
The latest compiler available in Visual Studio 15.3 with the version 14.11.25415.0 has support for the /std:c++17, which specifies that C++17 language features should be enabled. CMake should add /std:c++17 as a compiler option, if CXX_STANDARD is set to 17. It currently doesn't do that in CMake 3.9-rc4.