MUMBAI Lupin Ltd ( LUPN.NS ), India’s third-largest drugmaker by sales, is buying U.S. peer GAVIS Pharmaceuticals LLC for $880 million in its largest ever acquisition, seeking to revive its flagging growth in the United States, its largest market. The acquisition will give Mumbai-based Lupin access to a number of speciality generic drugs that tend to command a higher price as they are used to treat niche diseases