[Bug libstdc++/85672] [9 Regression] error: redefinition of 'constexpr long double std::abs(long double)'

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85672

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> (In reply to Marc Glisse from comment #4)
> > - -Wsystem-headers -Wundef will warn
> 
> That's the status quo. It would take a ton of effort to avoid -Wundef
> warnings in libstdc++ and that's not something I'm going to work on.

In other words, don't use -Wsystem-headers -Wundef