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

["mattyclarkson at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #9 from Matt Clarkson <mattyclarkson at gmail dot com> ---
(In reply to Jonathan Wakely from comment #7)
> GCC 7 now defines _GLIBCXX_RELEASE (with the same value as __GNUC__ has,
> i.e. the GCC major version, as an integer constant, but defined by the
> library headers not the compiler).
> 
> If that macro is present you have a version of libstdc++ that provides a
> working <regex>. To be really sure you could check it's defined to a value
> >= 5, in order to handle (hypothetical) unofficial 4.7 or 4.8 builds that
> have the patch backported.

Sweet! Will update the snippet on Stack Overflow. Thank you for this, you the
man ;)