fstatat: fix configuration bug on mingw, OpenBSD 4, Solaris 8

fstatat: fix configuration bug on mingw, OpenBSD 4, Solaris 8

* lib/fstatat.c (AT_FUNC_NAME): Use HAVE_FSTAT, not
HAVE_WORKING_FSTATAT_ZERO_FLAG, to decide whether to define
rpl_fstatat or fstatat.  This should fix the other problem
reported by Kai Habel in
<http://lists.gnu.org/archive/html/bug-gnulib/2011-11/msg00237.html>.
A similar problem was reported for OpenBSD 4.6 by Mats Erik Andersson
<http://lists.gnu.org/archive/html/bug-gnulib/2011-11/msg00239.html>
and I reproduced it on a Solaris 8 host we still have in production.