[Bug libstdc++/54924] Warn for std::string constructor with wrong size

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54924

--- Comment #11 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #10)
> I'm starting to think we should just make libstdc++ headers 100%
> warning-free and stop marking them as system headers with the pragmas. When
> they're installed under /usr they'd still be considered system headers
> though.

Invent #pragma not_system_header ? But being a system header is sometimes also
used to enable extensions, which may still be useful for libstdc++ headers.