Avoid using incomplete utimensat implementations.

At least NetBSD 6 has partial support for utimensat, implementing the
function but not providing needed definitions via sys/stat.h as POSIX
requires.  Check for needed defines, and if missing, undefine
HAVE_UTIMENSAT.

Signed-Off-By: Greg Troxel <gdt@ir.bbn.com>
Tested-By: Greg Troxel <gdt@ir.bbn.com>