[bug#60807] [PATCH v2] tests: reuse am_cv_filesystem_timestamp_resolution

    i'll note that Automake tests have been using `if ! ...`
    since 1.12 (2012),

The automake *tests* intentionally use modern shell syntax and
functionality, because they go to a lot of trouble to set up an
environment where those are supported, via test-init.sh. t/README talks
about this ("prefer using POSIX constructs ..."), and much else.
Some previous maintainers made that decision.

User-level scripts, non-test configure.ac's, etc., are another matter,
and should follow the "maximum portability" rules. --thanks, karl.