[Bug libstdc++/124268] `import std` initializer symbol is not shipped in `libstdc++`

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124268

--- Comment #17 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Conceptually, that would be no different to adding an autoconf check to see
whether the names from fenv.h (or setjmp.h, as in Bug 124554) can be exported,
and then guarding the exports with a configure macro. But automatic, without
having to add configure macros for everything which might not be exportable on
some target somewhere.