Far as I can figure, it is a circular sentence. How are you going to close the US to get a wall built if the wall is necessary to close the country?
What is really going on? He's trying to reactivate his people. Many of them will be realising he's all hat and no cattle. The wall is an appeal to their instinctive nationallism.
Many of them are low information voters. They probably believe him when he tells them he's started building it already.