April 22 (Reuters) - Tyman Plc, a supplier of doors and windows, will be acquired by U.S. company Quanex Building Products in a deal valuing the British firm at about 788 million pounds ($975.6 million), the companies said on Monday. ($1 = 0.8077 pounds) (Reporting by Aby Jose Koilparambil in Bengaluru; Editing by Savio D'Souza)