Avoid symbol clash for Xregion functions

We might be linking our Xregion and libX11 into the same binary,
causing a clash between the symbols. In theory they should be
compatible, but that's not guaranteed. Avoid the issue by adding
a "vnc" prefix to our version.

common/Xregion/Xutil.h

@@ -52,6 +52,22 @@ SOFTWARE.
 /* You must include <X11/Xlib.h> before including this file */
 #include "Xlib.h"
 
+/****** Avoid symbol clash with "real" libX11 ******/
+#define XClipBox vncXClipBox
+#define XCreateRegion vncXCreateRegion
+#define XDestroyRegion vncXDestroyRegion
+#define XEmptyRegion vncXEmptyRegion
+#define XEqualRegion vncXEqualRegion
+#define XIntersectRegion vncXIntersectRegion
+#define XOffsetRegion vncXOffsetRegion
+#define XPointInRegion vncXPointInRegion
+#define XPolygonRegion vncXPolygonRegion
+#define XRectInRegion vncXRectInRegion
+#define XShrinkRegion vncXShrinkRegion
+#define XSubtractRegion vncXSubtractRegion
+#define XUnionRectWithRegion vncXUnionRectWithRegion
+#define XUnionRegion vncXUnionRegion
+#define XXorRegion vncXXorRegion
 
 /*
  * opaque reference to Region data type