Bug fix from Zak Kipling <zak.kipling@cantab.net>

If you execute jam with no explicit target specified, it exits with
"EXITOK" regardless of whether the build actually succeeds...


[SVN r16336]

tools/build

@@ -1 +1 @@
-Subproject commit 4346e3c71794fe1be22e4f3e7c4a9cd3c3c8dab9
+Subproject commit ce28e203fb0a56f336137c869555999eed86e9d8