[Bug libstdc++/93469] memory header fails to compile with _XOPEN_SOURCE macro defined and -std=c++2a option specified

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93469

--- Comment #18 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(as a workaround in libstdc++)