Xbox GDKX support (#5869)

* Xbox GDK support (14 squashed commits)

* Added basic keyboard testing

* Update readme

* Code review fixes

* Fixed issue where controller add/removal wasn't working (since the device notification events don't work on Xbox, have to use the joystick thread to poll XInput)

src/haptic/SDL_syshaptic.h

@@ -26,6 +26,10 @@
 
 #include "SDL_haptic.h"
 
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 struct haptic_effect
 {
@@ -203,6 +207,11 @@ extern int SDL_SYS_HapticUnpause(SDL_Haptic * haptic);
  */
 extern int SDL_SYS_HapticStopAll(SDL_Haptic * haptic);
 
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* SDL_syshaptic_h_ */
 
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