[BioC] warning if pdf of vignette is missing

[Jeff Gentry]

> I have a different opinion.  Vignettes are an important component of
> BioC packages.  If R CMD build isn't building them, that usually
> indicates a problem that needs fixing.

It depends on the situation, that warning can mean various things.  It is
entirely likely that the vignettes can and do build when one gets that
warning (and this is the situation that I think Wolfgang was referring
to).  A few versions of R back 'check' started looking to make sure a .pdf
was *already* present in /inst/doc for every vignette file.  Personally I
don't really agree w/ that behavior but I don't make the rules. :)  

That was actually part of the reason behind all the pre-building/etc
before checks are run in the daily package checking code (which I'm
guessing has changed radically since I last saw it, but also guessing the
general flow of activity is similar in this regard).

-J