libstdc++11 not available on MacOS

Trying to build the current master on MacOS fails with the error

-- Conan: Automatic detection of conan settings from cmake
-- Conan: Settings= -s;build_type=Release;-s;compiler=apple-clang;-s;compiler.version=12.0;-s;compiler.libcxx=libc++;-s;compiler.libcxx=libstdc++11
-- Conan executing: conan install /Users/jean/CLionProjects/corsika/conanfile.txt -s build_type=Release -s compiler=apple-clang -s compiler.version=12.0 -s compiler.libcxx=libc++ -s compiler.libcxx=libstdc++11 -g=cmake --build=missing --build=bison
-- ERROR: Invalid setting 'libstdc++11' is not a valid 'settings.compiler.libcxx' value.
Possible values are ['libstdc++', 'libc++']
Read "http://docs.conan.io/en/latest/faq/troubleshooting.html#error-invalid-setting"

Commit 30d083aa introduced libstdc++11 as the compiler.libcxx setting, which even overrides my local profile properties (as one can see from the arguments in the setting list). However, libstdc++11 is not available on MacOS.

If possible, maybe it would be a good idea to include a runner in the CI that works with MacOS - at least if we want to continue MacOS support.