cmake: require VTK

CMakeLists.txt

@@ -100,7 +100,7 @@ if(CM_ALL_VTK_OUTPUT)
             )
         endif()
     endif()
-    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/Config.cmake.in "find_dependency(VTK)\n")
+    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/Config.cmake.in "find_dependency(VTK REQUIRED)\n")
 endif(CM_ALL_VTK_OUTPUT)
 
 if(CM_ALL_EXTERNAL_VORONOI)