(enum event_kind): Added new WHEEL_EVENT event.

Declare MOUSE_WHEEL_EVENT only if MAC_OSX defined.

src/termhooks.h

@@ -240,14 +240,27 @@ enum event_kind
 				   the mouse click occurred in.
 				   .timestamp gives a timestamp (in
 				   milliseconds) for the click.  */
-#if defined(WINDOWSNT) || defined(MAC_OSX)
-  MOUSE_WHEEL_EVENT,		/* A mouse-wheel event is generated
-				   on WINDOWSNT or MAC_OSX by a
-				   wheel on a mouse (e.g., MS Intellimouse).
-				   The event contains a delta that corresponds
-				   to the amount and direction that the wheel
-				   is rotated.  This delta is typically
-				   used to implement a scroll or zoom.
+  WHEEL_EVENT,			/* A wheel event is generated by a
+				   wheel on a mouse (e.g., MS
+				   Intellimouse).
+				   .modifiers holds the rotate
+				   direction (up or down), and the
+				   state of the modifier keys.
+				   .x and .y give the mouse position,
+				   in characters, within the window.
+				   .frame_or_window gives the frame
+				   the wheel event occurred in.
+				   .timestamp gives a timestamp (in
+				   milliseconds) for the event.  */
+#if defined(MAC_OSX)
+  MOUSE_WHEEL_EVENT,		/* A mouse-wheel event is generated on
+				   MAC_OSX by a wheel on a mouse
+				   (e.g., MS Intellimouse).  The event
+				   contains a delta that corresponds
+				   to the amount and direction that
+				   the wheel is rotated.  This delta
+				   is typically used to implement a
+				   scroll or zoom.
 				   .code gives the delta.
 				   .modifiers holds the state of the
 				   modifier keys.