Hi,
With latest ROOT version you should use following command for ROOT compilation:
cmake -DCMAKE_CXX_STANDARD=17 ...
Please check that is output of the command:
root-config --cflags
If ROOT compiled with default settings, c++11 will overwrite your c++17 flags.
You also can move -std=c++17 after root-config --cflags.
Regards,
Sergey