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

[redi at gcc dot gnu.org]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|8.0                         |7.0

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
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.