Review VTK external project definition

Define INCLUDE_DIRS, LIBRARY_DIRS and FIND_PACKAGE_CMD so that the build
system can automatically handle the dependencies.

CMakeExternals/VTK.cmake

@@ -62,6 +62,7 @@ IF(${add_project} OR CTK_LIB_Scripting/Python/Core_PYTHONQT_USE_VTK)
     ctkMacroEmptyExternalProject(${proj} "${proj_DEPENDENCIES}")
   ENDIF()
 
+  SET(${VTK_enabling_variable}_LIBRARY_DIRS VTK_LIBRARY_DIRS)
   SET(${VTK_enabling_variable}_INCLUDE_DIRS VTK_INCLUDE_DIRS)
   SET(${VTK_enabling_variable}_FIND_PACKAGE_CMD VTK)
 ENDIF()

Libs/Visualization/VTK/Widgets/CMakeLists.txt

@@ -1,16 +1,6 @@
 PROJECT(CTKVisualizationVTKWidgets)
 
 #
-# 3rd party dependencies
-#
-
-FIND_PACKAGE(VTK)
-IF(NOT VTK_FOUND)
-  MESSAGE(FATAL_ERROR "error: VTK package is required to build ${PROJECT_NAME}")
-ENDIF()
-INCLUDE(${VTK_USE_FILE})
-
-#
 # See CTK/CMake/ctkMacroBuildLib.cmake for details
 #