use find_file and not just a set for USE_VTK_FILE

Modules/FindVTK.cmake

@@ -67,7 +67,7 @@ IF (USE_INSTALLED_VTK)
   ENDIF (VTK_INSTALL_PATH)
 ELSE (USE_INSTALLED_VTK)
   IF (VTK_BINARY_PATH)
-    SET (USE_VTK_FILE ${VTK_BINARY_PATH}/UseVTK.cmake)
+    FIND_FILE(USE_VTK_FILE UseVTK.cmake ${VTK_BINARY_PATH})
   ENDIF (VTK_BINARY_PATH)
 ENDIF (USE_INSTALLED_VTK)