Use system doctest for building unit tests The embedded doctest has multiple problems since the last upstream release of box2d. E.g.: - https://github.com/erincatto/box2d/pull/682 - https://github.com/erincatto/box2d/issues/677
Make build reproducible Doxygen generated some files with names that had the absolute built path in their file names. By setting the STRIP_FROM_PATH variable, this can get replaced with locations relative to the source dir.
