Nick Bowler helped educate me on aclocal and libtool interaction on the Automake mailing list. The reason I was hitting this problem was aclocal was not finding libtool installed to the same prefix as automake. To test the prerelease automake I was leaving the base version of automake installed and installing the prerelease version to a prefix earlier on my $PATH, not realizing I also needed to install libtool to the same prefix or take other steps to help aclocal find the libtool macro definitions. After installing libtool to the same prefix, the problem disappeared.