This CMake module used to be provided by KDELibs4Support. Since the removal of that dependency from kio-extras, the module is not found and the SMB ioslave cannot be built.
This differential takes a local copy of the module from KDELibs4Support in kio-extras. No other part of KDE frameworks or Plasma appears to need Samba, therefore this approach should be preferable to copying it into ECM.