Update ExternalProjectDependency system based on commontk/Artichoke@70ed903

$ git shortlog cfefc7ea27..70ed903
Jean-Christophe Fillion-Robin (6):
      Properly exclude optional dependency when not only associated with top-level
      Remove unused test files
      Fix CMakeLists in MultiCall test to work as expected in the inner build case
      Properly set or restore the project variable. Fixes #4
      Make sure optional external project are not added.
      Rename "superbuild_message" to match ExternalProject naming convention
Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Update ExternalProjectDependency system based on commontk/Artichoke@70ed903