I switched from 

  target_compile_features(<target> PUBLIC css_std_17)

to

  set(CMAKE_CXX_STANDARD 17)
  set(CMAKE_CXX_STANDARD_REQUIRED ON)
  set(CMAKE_CXX_EXTENSIONS OFF)   # optional: disables GNU extensions

Which is now global in the CMakeLists.txt, so we only have to deal with one location if we want to change it. It also turns off the g++ options. On my
Mac I get now a clean

  -std=c++17

Please everybody test on your side. Change is committed.

Stefan