Display optional project only when configuring top-level project

See #418

CMake/ctkMacroCheckExternalProjectDependency.cmake

@@ -519,7 +519,9 @@ macro(superbuild_include_dependencies)
         if(_include_project)
           list(APPEND ${SUPERBUILD_TOPLEVEL_PROJECT}_DEPENDENCIES ${possible_proj})
         else()
-          superbuild_message(STATUS "${possible_proj}[OPTIONAL]")
+          if(${SUPERBUILD_TOPLEVEL_PROJECT}_SUPERBUILD)
+            superbuild_message(STATUS "${possible_proj}[OPTIONAL]")
+          endif()
         endif()
       endif()
     endforeach()