[Bug c++/79433] __has_include reports wrong result for std headers that #error on __cplusplus

[marc.mutz at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79433

--- Comment #5 from Marc Mutz <marc.mutz at kdab dot com> ---
Andrew, you're taking the easy way out. It might be that it works as
implemented, but that behaviour is useless.

Please explain how one should detect, in a portable way, whether string_view
and experimental::string_view is available, if not by headers check.