clock_gettime and CLOCK_REALTIME

[Eli Zaretskii]

Gnulib's gettime.c assumes that CLOCK_REALTIME is a preprocessor
macro:

  # if defined CLOCK_REALTIME && HAVE_CLOCK_GETTIME
    if (clock_gettime (CLOCK_REALTIME, ts) == 0)
      return;
  # endif

Does Posix mandate that CLOCK_REALTIME be a macro?  With mingw.org's
MinGW runtime 5.1.0, it isn't:

  typedef struct __clockid__ *clockid_t;
  extern clockid_t CLOCK_REALTIME;
  extern clockid_t CLOCK_MONOTONIC;

Why does Gnulib's gettime.c insist on CLOCK_REALTIME being a macro?

Thanks.