The current detection method fails on GCC 12.2 with -std=c++20 because
the __cpp_lib_span macro is undefined.
As per https://en.cppreference.com/w/cpp/utility/feature_test ,
__cpp_lib_span requires including either <version> or <span>.
Since both these headers were added in C++20, checking for C++20 is
sufficient (and simpler than using the library feature-test macro).
Signed-off-by: Bernie Innocenti <bernie@codewiz.org>
Co-authored-by: Derek Bailey <derekbailey@google.com>
