diff -r 0e4d8aa61ad7 -r c1b5d6e6ff74 src/Pure/Tools/build.scala
--- a/src/Pure/Tools/build.scala	Sat Nov 20 20:42:41 2021 +0100
+++ b/src/Pure/Tools/build.scala	Sun Nov 21 17:42:11 2021 +0100
@@ -305,7 +305,7 @@
     val log =
       build_options.string("system_log") match {
         case "" => No_Logger
-        case "true" => Logger.make(progress)
+        case "-" => Logger.make(progress)
         case log_file => Logger.make(Some(Path.explode(log_file)))
       }