getcwd: don't fail in a deep directory on a system without openat

getcwd: don't fail in a deep directory on a system without openat

Before this change, getcwd would fail when called from a directory
of depth PATH_MAX / 3 or greater.  That was due to the fact that
the non-openat implementation used "..", "../..", "../../..", etc.
to access ancestor directories.  With too many, that string would
be longer than PATH_MAX.
* lib/getcwd.c (HAVE_OPENAT_SUPPORT): Define also when we are
using gnulib's openat replacement.
* m4/openat.m4: Set GNULIB_OPENAT, so getcwd.c knows when
we're using the replacement function.