proc with process jobs enabled when appropriate, and defaulting delegate_ctlc to True.

Enable process jobs to ensure accurate determination of process completion in the presence of exec(3) on Windows.

Unfortunately the process job support is badly broken in process releases prior to 1.6.9, so we disable it in these versions, despite the fact that this means we may see sporadic build failures without jobs.

On Windows 7 or before the jobs are disabled due to the fact that processes on these systems can only have one job. This prevents spawned process from assigning jobs to its own children. Suppose process A spawns process B. The B process has a job assigned (call it J1) and when it tries to spawn a new process C the C automatically inherits the job. But at it also tries to assign a new job J2 to C since it doesn’t have access J1. This fails on Windows 7 or before.