automake: a millisecond is too fast for subsecond-mtime.

automake: a millisecond is too fast for subsecond-mtime.

This patch is from https://bugs.gnu.org/68325.

* m4/sanity.m4 (_AM_FILESYSTEM_TIMESTAMP_RESOLUTION): don't try
for a millisecond; make a hundredth of a second the fastest we'll go.

Apparently there are plenty of systems which supposedly support
subsecond-mtimes down to the millisecond and yet randomly
fail parallelized tests. For example:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=68119#18
Maybe the tests themselves need fixing. (The ones that run
autotools multiple times in succession.) A work in progress.