GABORONE, Nov 10 (Reuters) - Botswana is working on acquiring a majority stake in De Beers, President Duma Boko said on Monday, after Angola announced a rival bid to control the giant diamond company. Botswana, which owns 15% of De Beers and contributes 70% of its annual rough diamond production, considers the company a strategic national asset, despite a slump in global diamond prices that has hurt its economy.