Unescaped left brace in regex is illegal here in regex; marked by <-- HERE in m/\${ <-- HERE ([^ \t=:+{}]+)}/ at ./bin/automake.tmp line 3938.

goto line :3938 and change
$text =~ s/\${([^ \t=:+{}]+)}/substitute_ac_subst_variables_worker ($1)/ge;
to
$text =~ s/\$\{([^ \t=:+\{\}]+)\}/substitute_ac_subst_variables_worker ($1)/ge;
vi ./build_dir/host/automake-1.15/bin/automake.in

The error is with automake and perl v5.26.
In perl v5.22, using a literal { in a regular expression was deprecated, and will emit a warning if it isn’t escaped: \{. In v5.26, this won’t just warn, it’ll cause a syntax error.