automake: recommend autoreconf -f with version mismatches. From https://bugs.gnu.org/68674. * bin/automake.in (scan_autoconf_traces): change error to suggest running autoreconf -f. The version mismatch is most likely to happen due to the cache files, which aclocal && automake (the previous recommendation) does not delete. (scan_autoconf_files): likewise. * doc/automake.texi (Auto-generating aclocal.m4): mention autoreconf. Other minor wording changes.