utimensat: Work around trailing slash bug in utimensat() on AIX 7.2.
* m4/utimensat.m4 (gl_FUNC_UTIMENSAT): Require AC_CANONICAL_HOST. Add a
test for trailing slash handling. Improve cross-compilation guesses.
Conditionally define HAVE_NEARLY_WORKING_UTIMENSAT.
* lib/utimensat.c (rpl_utimensat): Add alternative implementation when
HAVE_NEARLY_WORKING_UTIMENSAT is defined.
* lib/utimens.c: Use the overridden utimensat when
HAVE_NEARLY_WORKING_UTIMENSAT is defined.
* doc/posix-functions/utimensat.texi: Mention the AIX bug.