docs: make dist implies make dvi.

docs: make dist implies make dvi.

This change handles https://bugs.gnu.org/7994.

* doc/automake.texi (Preparing Distributions): make distcheck
runs make dvi.
(Auxiliary Programs) <texinfo.tex>: mention
that make dist runs make dvi, and therefore a TeX system is
required when Texinfo sources are present. Add @cmindex entries
for all auxiliary programs while we're here.