Add possibility to skip building tests and examples

If the options BUILD_TESTING and BUILD_EXAMPLES are set to OFF (or --disable-tests and --disable-examples from dunecontrol), tests and examples (respectively) is not built.

During development iterations you typically want to disable one or both of these to save time. In particular, this can be useful when doing a `git bisect`.

The default for both options is still ON, so there should be no visible change if you don't update your options file (or reconfigure).