test: silent-defaults.sh rerun autotools each time. Follow-up to https://bugs.gnu.org/32868. * t/silent-defaults.sh: rerun $ACLOCAL && $AUTOMAKE for each trial, else test failed. (Why it has always succeeded before now, and/or what has changed, I don't know.) Remake the cache for each trial, for reliability. Echo trial description. * doc/automake.texi (Silent Rules): tweak text. (Unsilencing Automake): new node. Mention make --debug=p to completely unsilence rules.