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

[mattyclarkson at gmail dot com]

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

--- Comment #2 from Matt Clarkson <mattyclarkson at gmail dot com> ---
Because wehen I compile with clang against the libstdc++ the problem will still
occur and __GNUC__ will not be defined. This happens on any distro where GCC is
the default but ships clang as an alternative compiler.