2012-08-03 ago more informative process exit code;
2012-07-26 ago remove old output heaps, to ensure that result is valid wrt. check_stamps;
2012-07-23 ago added system build mode: produce output in ISABELLE_HOME;
2012-07-21 ago tuned -- no dependency on exit function;
2012-07-20 ago further imitation of "usedir" shell script;
2012-07-20 ago simplified script to build Isabelle/ML;