setting c++ standard from target_compile_option goes for higher standard

[ben.boeckel (Ben Boeckel (Kitware))]

@adaldev

An “At least” option should be better. When one is checking a tool or dependency version, he has “==”, “more” or “less” option (in various flavor), hasn’t he?

Yes, however this ends up just setting up update hazards for the future. You end up having to cut a new release just to support new dependencies (that upgrade on their own schedule). If older compilers matter, test them directly in CI. VTK, ParaView, and CMake all have builds that actually use the old compilers instead of relying on new compilers being told “use an older standard”. Bug fixes get backported to old standards in new compilers; it doesn’t meant that old compilers actually work.