[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 #4 from Matt Clarkson <mattyclarkson at gmail dot com> ---
That's OK. I'm not particularly looking for the macro to be backported to 4.9.
Just as we move forward the new macro is available. If not it's not the end of
the world I can always maintain the snippet internally.