[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 #1 from Marc Mutz <marc.mutz at kdab dot com> ---
And no, checking __cplusplus in addition is not an option, since many
compilers, GCC included (__cplusplus==1, remember?), do not necessarily bump
__cplusplus when they implement enough core features to make something like
string_view (which can be implemented in C++11 just fine) work.