diff -r 067a01827fca -r 8e61560ded89 src/Pure/System/cygwin.scala
--- a/src/Pure/System/cygwin.scala	Sat Apr 17 20:42:26 2010 +0200
+++ b/src/Pure/System/cygwin.scala	Sat Apr 17 21:01:55 2010 +0200
@@ -91,9 +91,9 @@
 
   def check_root(): String =
   {
-    val root_env = java.lang.System.getenv("CYGWIN_ROOT")
+    val this_cygwin = java.lang.System.getenv("THIS_CYGWIN")
     val root =
-      if (root_env != null && root_env != "") root_env
+      if (this_cygwin != null && this_cygwin != "") this_cygwin
       else
         query_registry(CYGWIN_SETUP1, "rootdir") orElse
         query_registry(CYGWIN_SETUP2, "rootdir") getOrElse