Big Bob Gibson Bar-B-Q in Decatur, Alabama, is dubbed “home of white barbecue sauce”—a tangy concoction based on mayonnaise and cider vinegar with a healthy dose of black pepper. Here’s my take with a cut-up chicken, so there are plenty of nooks and crannies for the sauce to season. If you’re using charcoal, you can throw a couple of chunks of hickory w…