Add a project configuration option to achieve the following behaviour (or keep
the normal).
Project A (downstream) depends on project B (upstream).
When a build of B is running and a build of A is scheduled, A waits in the queue
(as if there were another build of A).
This behaviuor is needed to avoid building A (in parallel with B already
running) and then rebuild A after B has completed.

Hudson already checks if a project in the build queue is already running, can it
check also if a build of an upstream project is already running?