This fails on ash, dash, and busybox sh (for now). But it's kind of
important since $HOME often has spaces on Win32. I hope dash will be fixed
relatively soon.
I downgraded it to a warning since on Unix, this probably isn't a problem.
This includes a fairly detailed test of various known shell bugs from the
autoconf docs.
The idea here is that if redo works on your system, you should be able to
rely on a *good* shell to run your .do files; you shouldn't have to work
around zillions of bugs like autoconf does.