Fix VS2017 build
Fix VS2017 build

Explicitly enable C++17 language features on Visual Studio 2017 to fix
C2429 (use of 'terse static assert' in commit e35a41e) on AppVeyor CI.

bkircher/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,otris/ews-cpp