e.g. the /bin/sh on my Ubuntu 7.04 system

Fixes a problem where /dev/null doesn't exist on mingw.

os.spawnv is broken on non-cygwin Windows (looks like it goes via a shell).

Also the test driver doesn't understand 'cmm' files
so something will have to be written before these tests
can even run.

It breaks some tests on Windows, because the stage=2 setting is lost
in the recursive call.  I don't understand why these variables are
passed in MAKEFLAGS on Windows (cygwin) but not on Unix, something to
do with environment variables presumably.