graphviz: define librsvg as optional

If `librsvg` formulae is already installed, configure finds it and the 'rsvg' Graphviz plugin is built. This fails when `gettext`'s keg-only **libintl** can't be found.

This fix adds and defines the `librsvg` dependency as `:optional`, though it could be `:recommended`. I could not find on the [Graphviz dependency listing](http://www.graphviz.org/Download_source.php), or in the [build doc](https://github.com/ellson/graphviz/blob/master/doc/build.html), which might be preferred. I think that plugin is heavily used, but have no real data to back it up.