src/Pure/Tools/build.scala

   {
     val build_options = Word.explode(Isabelle_System.getenv("ISABELLE_BUILD_OPTIONS"))
 
     var base_sessions: List[String] = Nil
     var select_dirs: List[Path] = Nil
-    var log: Logger = No_Logger
     var numa_shuffling = false
     var presentation = Presentation.Context.none
     var requirements = false
     var soft_build = false
     var exclude_session_groups: List[String] = Nil