[Bug libstdc++/71444] Error constants for MinGW-w64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71444

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In fact it looks like the patch should work for mingw-w64 v4, and maybe v3 too.