On macOS 10.13, I'm seeing test failures:

FAIL: test-futimens
===================

../../gltests/test-futimens.h:170: assertion 'ctime_compare (&st3, &st2) < 0' failed
FAIL test-futimens (exit status: 134)

FAIL: test-utimens
==================

../../gltests/test-utimens.h:149: assertion 'ctime_compare (&st3, &st2) < 0' failed
FAIL test-utimens (exit status: 134)

I looked at the file timestamps using coreutils' 'stat' program. These two
failures are apparently caused by st_ctime being unchanged when you ask
for an st_atime change using utimensat(). I think this is not worth working
around, because every read access on a file causes st_atime to change without
st_ctime changing.

Another test failure, then:

./test-utimens.h:164: assertion 'func (BASE "link/", NULL) == -1' failed
Abort trap: 6


2021-01-02  Bruno Haible  <bruno@clisp.org>

	utimens: Fix test failure on macOS 10.13.
	* lib/utimens.c (fdutimens): Before calling utimensat, recognize a
	filename ending in a slash that does not point to a directory.

2021-01-02  Bruno Haible  <bruno@clisp.org>

	utimens: Avoid test failures on macOS 10.13.
	Reported by Martin Storsjö <martin@martin.st> in
	<https://lists.gnu.org/archive/html/bug-gnulib/2020-12/msg00003.html>.
	* tests/test-utimens-common.h (check_ctime): Define to -1 on macOS.
	* tests/test-utimens.h (test_utimens): Don't expect a ctime change when
	only the atime is requested to change.
	* tests/test-futimens.h (test_futimens): Likewise.
	* tests/test-lutimens.h (test_lutimens): Likewise.