[Bug libstdc++/78905] Add a macro to determine that the library is implemented

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78905

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Why don't you use:
__GNUC__ >=5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)

instead for checking GCC version?