SlicerRT extension build failure

Any idea why this is happening?

    -- SuperBuild - First pass - done
    -- SuperBuild - inner => Requires Plastimatch, 
    -- SuperBuild -   Plastimatch[OK]
    CMake Error at SuperBuild/External_Plastimatch.cmake:46 (ExternalProject_Add):
      Unknown CMake command "ExternalProject_Add".
    Call Stack (most recent call first):
      /home/kitware/Dashboards/Package/Slicer-481/CMake/ExternalProjectDependency.cmake:683 (include)
      /home/kitware/Dashboards/Package/Slicer-481/CMake/ExternalProjectDependency.cmake:746 (ExternalProject_Include_Dependencies)
      SuperBuild.cmake:24 (ExternalProject_Include_Dependencies)
      CMakeLists.txt:43 (include)

Probably include(ExternalProject) is missed.

Yes, that is what a topic on Stack Exchange suggests. It is easy and harmless to add. But I have doubts on effectiveness because it was apparently never needed before, and when I build it locally, I don’t get this error.

Explicitly including the ExternalProject module is only required with Slicer 4.8.1. Since r26984 it is included through UseSlicer.cmake. This is why it was removed in SlicerRt/SlicerRT@6495332

It seems the s4ext points to an older version. Could someone please update when you get a chance? Thanks!