Jake Gagne, 2, rides a pony at the Texas Farmers’ Market at Mueller last Sunday in Austin. (Photo ©Gary Jacobson/Texas BBQ Posse) We ate prime rib and smoked chicken under a historic airport hangar at a mid-week farmers market while, not far away, a singer crooned an Elvis tune. Nearby, kids and adults had their pictures taken atop a giant Longhorn, which was saddled and very much alive. Wise men say this must be Austin. Since moving to ATX a couple years ago, we’ve been regulars at the Sunday pop-up market on the site of the old Mueller Airport, just…