    % scons -Q --debug=stacktrace
    scons: *** [prog.o] Source `prog.c' not found, needed by target `prog.o'.
    scons: internal stack trace:
      File "SCons/Job.py", line 203, in start
        task.prepare()
      File "SCons/Script/Main.py", line 180, in prepare
        return SCons.Taskmaster.OutOfDateTask.prepare(self)
      File "SCons/Taskmaster.py", line 186, in prepare
        executor.prepare()
      File "SCons/Executor.py", line 418, in prepare
        raise SCons.Errors.StopError(msg % (s, self.batches[0].targets[0]))