Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Remove code related to CTK_EXTERNAL_LIBRARY_DIRS
  Automatically add CMAKE_GENERATOR external project argument
  Automatically add LIST_SEPARATOR external project argument
  Rename 'ctkMacroEmptyExternalproject' into 'superbuild_add_empty_external_project'
  Teach 'superbuild_include_dependencies' how to update <proj> variable.
  Rename 'ctkMacroCheckExternalProjectDependency'
  Generalize the logic allowing to decide if an external project is optional.
  Update indent variable based on external project stack size
  SuperBuild - Improve management of "proj" variable adding stack support
  Generalize how superbuild CMake variables can be passed to inner project.
  Add "superbuild_cmakevar_to_cmakearg" convenience function
  Introduce "SUPERBUILD_TOPLEVEL_PROJECT" project variable
  Simplify CTK external project specifying 'STEP_TARGETS'

Conflicts:
	CMake/LastConfigureStep/CTKGenerateCTKConfig.cmake