Switch to C++17 version of static_assert
Switch to C++17 version of static_assert

Message can be omitted with C++17. Compilers that pass
EWS_HAS_NON_BUGGY_TYPE_TRAITS should come with that version by now, I
guess.

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