Long gone are the days when galleries mounted summer group shows that were simply roundups of the dealers’ rosters. A high-concept entry to the annals of the more ambitious recent variety of summer group show is “Retrospective of S,” in which 10 artists selected by co-curator Samuel Messer have created works of art supposedly by S, a fictional artist of the curators’ creation. Wall labels are contributed by co-curator Jonathan Safran Foer.