test: check that subsecond mtime works with make.

test: check that subsecond mtime works with make.

Trying to fix https://bugs.gnu.org/68808.

* m4/sanity.m4 (_AM_FILESYSTEM_TIMESTAMP_RESOLUTION): check
that make works with subsecond mtimes, as well as sleep and ls.
(Known to fail with the make 3.81 that ships with macOS.)
* configure.ac (MTIME_RESOLUTION): notice if it erroneously
expands to the empty string, i.e., fall back to 1 if
$am_cv_filesystem_timestamp_resolution ended up being unset.
* t/ax/test-defs.in (MTIME_RESOLUTION): also fall back to 1
if the variable to not set, with a message.
Fix other typos en passant.
* HACKING: tweak.