There is no need to rebuild everything if only the documentation files
have changed.
Unfortunately there doesn't seem to be any way to do the same thing for
Travis CI yet, see https://github.com/travis-ci/travis-ci/issues/6301
Unlike Travis CI, Appveyor doesn't do this automatically, but we will
need them soon (the currently existing Catch submodule is not used by
Appveyor builds but, arguably, should be too).
