diff -r 5b49c650d413 -r 72900f34dbb3 src/Pure/Tools/build.scala
--- a/src/Pure/Tools/build.scala	Mon Jun 07 09:36:21 2021 +0200
+++ b/src/Pure/Tools/build.scala	Mon Jun 07 11:42:05 2021 +0200
@@ -296,7 +296,7 @@
     val log =
       build_options.string("system_log") match {
         case "" => No_Logger
-        case "-" => Logger.make(progress)
+        case "true" => Logger.make(progress)
         case log_file => Logger.make(Some(Path.explode(log_file)))
       }