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

[vital.had at gmail dot com via Gcc-bugs]

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

--- Comment #15 from Sergey Fedorov <vital.had at gmail dot com> ---
And this also:

```
#if !defined _POSIX_C_SOURCE && \
    defined(__APPLE__) && defined(__MACH__) && !defined(__clang__)
// Needed to prevent EOWNERDEAD issues with GCC on Mac
#define _POSIX_C_SOURCE 200809L
#endif
```

https://github.com/unicode-org/icu/blob/89fe16ef3b77a6b98f7664f52338c8ff80e28096/icu4c/source/common/uposixdefs.h