CMake: Prefer C++20 but minimum C++11

[cjee21]

No description provided.

[JeromeMartinez]

It seems to be C++20 or C+11, isn't it possible to select the latest C++ version available e.g. C++14 or C++17?

[cjee21]

It's supposed to degrade automatically if 20 is not available if I understand correctly how CMake works. Then if it drops below 11 which is the other condition then it will error. Not sure if this is compiler dependent.

[JeromeMartinez]

I think I merged too quickly, I like the automatic upgrade idea but it is too much binary, as we are there please a new PR with something like

function(enable_latest_cxx target)
    if(DEFINED CMAKE_CXX_STANDARD)
        # User override
        target_compile_features(${target} PUBLIC cxx_std_${CMAKE_CXX_STANDARD})
        return()
    endif()
    set(_known_standards 26 23 20 17 14 11)
    foreach(std ${_known_standards})
        if("cxx_std_${std}" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
            target_compile_features(${target} PUBLIC cxx_std_${std})
            return()
        endif()
    endforeach()
    message(FATAL_ERROR "Compiler does not support C++11 or newer")
endfunction()

and adapt MIL PR accordingly.