Comment 5 for bug 1853168

I think the clock_gettime() and CLOCK_REALTIME definitions are pulled in through
the following condition in time.h.

> /* POSIX 2008 says clock_gettime and timespec are defined in time.h header,
> but other systems - like Linux, Solaris, etc - tend to declare such
> recent extensions only if the following guards are met. */
> #if !defined(IN_WINPTHREAD) && \
> ((!defined(_STRICT_STDC) && !defined(__XOPEN_OR_POSIX)) || \
> (_POSIX_C_SOURCE > 2) || defined(__EXTENSIONS__))
> #include <pthread_time.h>
> #endif

https://github.com/mirror/mingw-w64/blob/3ac71ed3105c2989ba378e2e88d9405f65797178/mingw-w64-headers/crt/time.h#L302-L309