Fixed X11 XInput2 multitouch test to work in configure script.

configure wraps this C code in a main() function, so you can't declare a
function body in there. Besides, I'm not sure why we declared a function
that's part of the multitouch API anyhow.

Now we just reference a type that only exists if the headers have multitouch
support and call it a day.

configure

@@ -20173,11 +20173,6 @@ main ()
 
 int event_type = XI_TouchBegin;
 XITouchClassInfo *t;
-Status
-XIAllowTouchEvents(Display *a,int b,unsigned int c,Window d,int f)
-{
-    return (Status)0;
-}
 
   ;
   return 0;