Handle Graphviz capitalization changes

• Use the consistently-capitalized variables.
• Don't link_directories() with something that isn't defined by the find-module (anymore; it was done accidentally in the previous version of the find-module).
• Link to the named-variable for the Graphviz libraries, not the names of the libs (which cause -lgvc to show up in the linker command, which worked when the library-path was added to the link_directories, but not anymore; in any case we have the full paths for those libraries from the find-module).