add target_compile_features(... cxx_std_17) to generated CMakeLists.txt

[KrisThielemans]

We require C++-17, but it isn't encoded anywhere. https://stackoverflow.com/questions/70667513/cmake-cxx-standard-vs-target-compile-features#comment135186815_70667652 says that we should add target_compile_features(... cxx_std_17) , which I think should be in the OBJECT library.