[Bug libstdc++/65473] Including does not define __GLIBCXX__

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65473

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But there's no such thing as a detection macro in the standard, so it's not
going to talk about where they belong.