Compiler support for C++17

# Notes

As of 2020-11-20, the latest release of Oracle Developer Studio is 12.6. Its documentation does not mention C++17. Cray compiler may have support for some features earlier than 11.0. That version is when it became a derivative of Clang, gaining all of the attendant language feature support of the base compiler. See Cray/HPE document S-2179.