more robust fifo rendezvous: Cygwin 1.7 does not really block as expected;

--- a/src/Pure/System/isabelle_process.ML	Mon Aug 09 18:18:32 2010 +0200
+++ b/src/Pure/System/isabelle_process.ML	Mon Aug 09 21:23:24 2010 +0200
@@ -63,8 +63,10 @@
 fun rendezvous f fifo =
   let
     val path = File.platform_path (Path.explode fifo);
-    val result = f fifo;  (*block until peer is ready*)
-    val _ = OS.FileSys.remove path;  (*ready; prevent future access -- potential race condition*)
+    val result = f fifo;  (*should block until peer is ready*)
+    val _ =
+      if String.isSuffix "cygwin" ml_platform then ()  (*Cygwin 1.7: no proper blocking on open*)
+      else OS.FileSys.remove path;  (*prevent future access*)
   in result end;
 
 in