Use alarm() iff we're not on Windows.

Use alarm() iff we're not on Windows.

We assume "not on Windows" means "on UN*X", which means we have alarm()
(we don't care about V6 and earlier; 1978 called, they want their UNIX
back).  If somebody wants to support something that's neither UN*X nor
Windows, they'll need to contribute support, properly checking for that
other OS.