Thanks for doing this. I only recently learned about add_compile_options(/Zc:__cplusplus).
It is quite simply scandalous.

I'm a little nervous about using CMAKE_CXX_COMPILER_FRONTEND_VARIANT, since a Google of it does not point to the usual CMake doc page, though there are definitely references to it in various places. It appears to be undocumented -- is that right? If so, I think I'd prefer an equivalent PR that just adds the missing variant(s) for WIN32. If it is documented, and I just missed it, I'm all for the current approach, except:

Please don't leave a block of commented-out CMake in your PR. :)

If I type CMAKE_CXX_COMPILER_FRONTEND_VARIANT into google search, the first hit points to https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_FRONTEND_VARIANT.html, which I suppose is the official documentation.
I oversaw that it was introduced in version 3.14 which makes the adjustment of cmake_minimum_required(VERSION 3.8...3.20)necessary.

If this is a no-go, then

if (MSVC)
    get_filename_component(CNAME "${CMAKE_CXX_COMPILER}" NAME)
    message(STATUS "Detected MSVC style compiler frontend '" ${CNAME} "'")
    add_definitions(/W3)
    add_compile_options(/Zc:__cplusplus) # not sure, if it necessary for clang-cl
else()
    get_filename_component(CNAME "${CMAKE_CXX_COMPILER}" NAME)
    message(STATUS "Detected GNU style compiler frontend '" ${CNAME} "'")
    add_definitions(-Wall)
endif ()

is the way to go (if we assume, there exist only 2 different command-line styles)

Martin

Oh no I didn't mean you needed to revert it to an older-Cmake way of doing things. 3.14 is pretty old. I was just worried that CMAKE_CXX_COMPILER_FRONTEND_VARIANT was an undocumented/internal variable. Now that I see it's official, I'd like to use that version of your patch. Sorry for the confusion.

I've reverted it and updated the cmake_minimum_requiredlines

Martin

Very nice! Thanks again for doing this.