Autodesk Inc. said it agreed to buy Softdesk Inc. for about $72 million in stock. Softdesk, of Henniker, N.H., makes architectural and engineering software and had revenue of $26.5 million for the nine months ended Sept. 30. Autodesk, a San Rafael-based software maker, said it would issue 0.44 shares for each Softdesk share in the transaction. Based on closing stock prices Tuesday, the deal values Softdesk at $12 a share. Softdesk stock closed at $9.75, up 50 cents on the Nasdaq Stock Market. Autodesk finished at $26.63, down 62.5 cents on the Nasdaq. The deal, announced after markets closed, is expected to close by the end of April.