[BioC] Rgraphviz for Windows

[Jeff Gentry]

> graphviz.  Alternatively, you might be able to install cygwin and then 
> install graphviz on top of that (but I would suggest checking with 
> someone else before trying that option.)

The problem here is that cygwin and R don't play nicely together in these
situations.  Building Graphviz w/ cygwin causes problems when Rgraphviz
tries to link to it.