Fixed a Boost Jam bug causing it to sometimes trigger actions depending on targets that have not been built yet. Test case included. Updated related code comments.
Bug was happening when we had a multifile action that got triggered to build its non-initial target. Then while that action was being executed all the other targets were reporting as 'already built' and were getting used by other actions prematurely. Quick-fixed by making all targets built by a single action list each other as 'included' causing anything else depending on any of these targets to automatically depend on all the others in the group as well.
The solution is not perfect as it might have some unexpected interactions with other uses of 'included' targets and now if any target in a group is not up to date then all of them will be rebuilt even if actually did not need the target that was up to date. On the other hand this should be a really rare use case as it would require the one target in a group to be up to date and be needed while another in the same group (i.e. built by the same action) to not be up to date.
