(Adds Ackman comments) NEW YORK, Sept 11 (Reuters) - William Ackman, head of hedge fund firm Pershing Square Capital Management, said on Friday that stocks were cheap at current levels and that Mondelez International Inc could be an acquisition target. "I think stocks are pretty cheap," Ackman told cable business channel CNBC, but he excluded energy stocks since he was not an investor in the sector. He said he was "a little more bullish" on the stock market than fellow hedge... More