I'm attaching a patch to build.py to make it OS-independent. Basically, I've replaced all the "mv" and "rm" invocations with their stdlib's equivalents. I haven't touched any of those harmless os.system calls to Java and Maven though I have replaced calls to "python" with "sys.executable".
I haven't tested it on AS/400 etc. but it sure works on both Linux Ubuntu and Windows XP now, both using Python 2.5.
Diffed against SVN trunk, r500.