Some cleanups on the new XInput code.

One or two things got moved around, but largely this is hooked up correctly
 in the Unix configure system now: it can be dynamically loaded and fallback
 gracefully if not available, or libXi can be directly linked to libSDL.

XInput support can be --disable'd from the configure script, too (defaults to
 enabled).

Please note that while the framework is in place to gracefully fallback, the
 current state of the source requires XInput. We'll need to adjust a few
 things still to correct this.

src/video/x11/SDL_x11events.c

@@ -29,12 +29,6 @@
 #include "SDL_x11video.h"
 #include "../../events/SDL_events_c.h"
 
-extern int motion;              /* the motion event id defined by an XInput function */
-extern int button_pressed;      /* the button_pressed event id defined by an XInput function */
-extern int button_released;     /* the button_released event id defined by an XInput function */
-extern int proximity_in;        /* the proximity in event defined by an XInput function */
-extern int proximity_out;       /* the proximity out event defined by an XInput function */
-
 static void
 X11_DispatchEvent(_THIS)
 {