Produce a dependency graph based on a list
            of tasks produced by the parser.

def _build_graph(self):
        """ Produce a dependency graph based on a list
            of tasks produced by the parser.
        """
        self._graph.add_nodes_from(self.tasks)
        for node1 in self._graph.nodes:
            for node2 in self._graph.nodes:
                for input_file in node1.inputs:
                    for output_file in node2.outputs:
                        if output_file == input_file:
                            self._graph.add_edge(node2, node1)
        for order, task in enumerate(topological_sort(self._graph)):
            task.predecessors = self._graph.predecessors(task)
            task.order = order