Donations to Art Gallery Building Subscribers Legally Bind Es- tates and Executors to Fulfill Gifts.      On behalf of Mr. H.A. Stone, who subscribed $50,000 towards the endowment of a civic art gallery, Mr. W.H. Malkin has forwarded to Mayor L.D. Taylor a memorandum signed by all donors to the $100,000 fund, making their gifts binding on their estates and executors.      In an accompanying letter, Mr. Malkin explains the intent of the donors, and in the memorandum it is stipulated that the subscriptions are payable to the treasury of the art gallery when the latter is organized.