Macpage, a 100-employee accounting and consulting firm headquartered in South Portland, will merge with a nationwide company. The merger with Wipfli, which is based in Milwaukee, Wisconsin, was announced Wednesday. Macpage was founded in 1956. In addition to its headquarters in South Portland, the firm also has an office in Augusta and most of its clients are Massachusetts, New Hampshire and Maine. Wipfli, founded in 1930, has nearly 2,000 employees, 47 offices in the U.S.