[Bug libstdc++/57582] clone is effectively reserved by <iostream> and should not be

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57582

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. using `class clone*` to refer to the type works.

This is the same as needing to use `struct stat` to disambiguate the type from
the function.

If we stop defining _GNU_SOURCE that workaround wouldn't be needed.