[Bug c++/63287] __STDCPP_THREADS__ is not defined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63287

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> I'm not sure what the best way to meet that requirement is, given that .

Oops, I was going to say "... given that the availability of threads may not be
known until link-time, depending whether libpthread is used.