As far as I can understand it, only the Wii U GamePad is supported as a game controller in the Wii U version of SDL, with no handling for Wii Remotes, Wii Classic Controllers or Wii U Pro Controllers, or--more exotically--the Wii Balance Board. This makes SDL-based apps on Wii U significantly less usable than on other platforms, since they're forced to only support one of the four or so controller types most commonly used on Wii Us.