PSPP autobuilder updates

[Ben Pfaff]

I've added some small extra features to the PSPP autobuilder at
https://benpfaff.org/~blp/pspp-master/. First, that URL now has a
little documentation at the top. Second, the "latest*" links under it
are now redirects, which means that when you download them, you get a
better filename than just, say, "latest-source.tar.gz". Finally, I've
added a "latest-windows" redirector to make it easy to find the latest
Windows build.

I've also revised the documentation at
https://www.gnu.org/software/pspp/get.html to make it easier to find
what I think most users will want.

Best wishes,

Ben.