After four decades, the landmark Harvard Square restaurant Casablanca is about to close. It’ll be open through the end of September, but friends and fans of Sari Abul-Jubein’s place gathered Thursday for a little farewell bash. (Abul-Jubein has said he plans to spend a lot of time at his other Cambridge eatery, Casbah, which he is renaming Fat Man Café.) Whither Casablanca? Word is chef Dante DeMagistris , owner of Cambridge’s dante and Belmont’s Il Casale, may be interested in opening a new restaurant in the space.