[Bug c++/85269] warn for referenced standard symbols that aren't guaranteed to be declared in a header

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85269

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
LWG issues 343 has some bakckground into this problem:
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#343

It was resolved via LWG #1178 without solving the portability problem:
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#1178