ignore undefined environment;

--- a/src/Pure/System/standard_system.scala	Mon Dec 28 22:03:14 2009 +0100
+++ b/src/Pure/System/standard_system.scala	Mon Dec 28 22:57:37 2009 +0100
@@ -107,8 +107,10 @@
     for (s <- args) cmdline.add(s)
 
     val proc = new ProcessBuilder(cmdline)
-    proc.environment.clear
-    for ((x, y) <- env) proc.environment.put(x, y)
+    if (env != null) {
+      proc.environment.clear
+      for ((x, y) <- env) proc.environment.put(x, y)
+    }
     proc.redirectErrorStream(redirect)
 
     try { proc.start }