We would like to ensure that adopters of the new Ninja builder can use the same tool while working on backports or other maintenance tasks on older stable branches, so we should backport support as far as is possible and/or makes sense. At least, we should ensure that v4.2 offers it.

This isn't being filed as a backport ticket because it will be easier to just take the finished product, rather than try to backport all of the individual tickets that have gone into the new Ninja builder.