Adding _SC_MONOTONIC_CLOCK?

[walt]

The latest glib2 from pkgsrc fails because we don't define
_SC_MONOTONIC_CLOCK in unistd.h:

#if !defined(_POSIX_MONOTONIC_CLOCK) || _POSIX_MONOTONIC_CLOCK >= 0
      if (sysconf (_SC_MONOTONIC_CLOCK) >= 0)
        posix_clock = CLOCK_MONOTONIC;

According to the NetBSD sysconf manpage, _SC_MONOTONIC_CLOCK
should return the POSIX version number of the Monotonic Clock
option, or -1 if it's not available.

Adding that definition seems pretty harmless, right?  Is there
any reason to avoid adding it?