Add links to cairo pdf output
Here's a patch I used to test graphviz with cairo pdf links. PDF links feature is not yet in a stable release of cairo. Links are in the current cairo 1.15.8 snapshot. The patch is only a prototype I used for testing. It probably should check the size of the url.
Also, the no white background option should be enabled for PDF. PDFs have a transparent background. The viewer will always render on top of a white background. The transparent background is useful in case you want to use a PDF file like an EPS file.