The U.S. economy, held back by the summer drought, expanded at a moderate 2.6 percent annual rate in the July-September quarter, the government said today. The Commerce Department said the performance of the gross national product, the broadest measure of economic health, slowed in the third quarter after growing at an annual rate of 3.2 percent during the first six months of the year. It was the slowest pace since the fourth quarter of 1986.