Add steam deck detection and HIDAPI driver scaffold.

include/SDL_hints.h

@@ -943,6 +943,17 @@ extern "C" {
  */
 #define SDL_HINT_JOYSTICK_HIDAPI_STEAM "SDL_JOYSTICK_HIDAPI_STEAM"
 
+/**
+ *  \brief  A variable controlling whether the HIDAPI driver for the Steam Deck builtin controller should be used.
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - HIDAPI driver is not used
+ *    "1"       - HIDAPI driver is used
+ *
+ *  The default is the value of SDL_HINT_JOYSTICK_HIDAPI
+ */
+#define SDL_HINT_JOYSTICK_HIDAPI_STEAMDECK "SDL_JOYSTICK_HIDAPI_STEAMDECK"
+
 /**
  *  \brief  A variable controlling whether the HIDAPI driver for Nintendo Switch controllers should be used.
  *