utimens: fix regression on Solaris

utimens: fix regression on Solaris

Revert commit 26c5fd742f.  Solaris lacks futimens and futimes, so
futimesat is the only way to change fd timestamps.  But since
FreeBSD futimesat can't change fd timestamps, we need a configure
check to avoid the crash there.

* m4/utimens.m4 (gl_UTIMENS): Check for BSD bug.
* lib/utimens.c (fdutimens): Revert 2009-11-08 change; Solaris 10
can only change fd timestamps via futimesat.  Instead, use an
additional witness macro to avoid BSD bug.
Reported by Jim Meyering.

Signed-off-by: Eric Blake <ebb9@byu.net>