Build hidapi code into SDL as a new public API

This prevents conflicts with hidapi linked with applications, as well as allowing applications to make use of HIDAPI on Android and other platforms that might not normally have an implementation available.

src/hidapi/mac/hid.c

@@ -21,8 +21,6 @@
  ********************************************************/
 #include "../../SDL_internal.h"
 
-#ifdef SDL_JOYSTICK_HIDAPI
-
 /* See Apple Technical Note TN2187 for details on IOHidManager. */
 
 #include <IOKit/hid/IOHIDManager.h>
@@ -1191,5 +1189,3 @@ int main(void)
 	return 0;
 }
 #endif
-
-#endif /* SDL_JOYSTICK_HIDAPI */