From an office suite on the 26th floor of the iconic Frost Bank Tower in Austin, Texas, a little-known recruiting firm called Crossover is searching the globe for software engineers. Crossover is looking for anyone who can commit to a 40- or 50-hour workweek, but it has no interest in full-time employees. It wants contract workers who are willing to toil from their homes or even in local cafes. “The best people in the world aren’t in your Zip code,” says Andy Tryba, chief executive of Crossover, in a promotional YouTube video. Which, Tryba emphasizes, also means you don’t have...