bug#8289: distcheck and make dvi

[Karl Berry]

Hi Ralf and all,

make distcheck runs make dvi.  If an author has only made .pdf's with
images, this will cause a surprising error that .eps files are missing.
(Someone just wrote me about this.)

I see the issue has been noted before, e.g.:
http://comments.gmane.org/gmane.comp.sysutils.automake.general/10304

I suggest that the workaround Ralf suggests there -- an empty make rule
for dvi: -- be explicitly documented.  Another (often preferable) option
is to create and distribute the .eps files, of course.

Another possible change, in addition to messing with the documentation,
would be to make the "dvi" a variable that such non-dvi-generating
people can override with "pdf" if they wish.  Then they could get the
benefit of the check in a clean way without forcing .eps.  Or maybe
dvi: pdf
would have the same effect?  I'm not sure.

Thanks,
Karl