When building the documentation via `make doc`, it's necessary to have [Doxgen](https://www.doxygen.nl/index.html) installed as well as the `dot` tool from [Graphviz](http://www.graphviz.org/) to generate class hierarchy graphs that are added to the class documentation pages. If `dot` is not present, the configuration process succeeds but a large number of errors are generated when the documentation build process attempts to call `dot` many times to generate the graphs and fails because the command is missing. The build still ultimately completes but it would be good to remove these errors. From discussion with @dmoxey, a couple of options seem to be available here: 1) Make `dot` a requirement and make the configuration fail if it's missing 2) Keep `dot` optional but update the `Doxyfile.in` file to set `HAVE_DOT` and `CLASS_GRAPH` to `NO` when it is not found which prevents the documentation build attempting to run dot to create the class graphs. The aim is to go with option (2).