[Python-Dev] Fixing buildbot/external(-amd64).bat files on Windows

["Martin v. Löwis"]

> I don't know how the existing vcbuild line ever worked, given the following output from vcbuild /?:
> 
> Usage: vcbuild [options] [project|solution] [config|$ALL]

It didn't. All the existing slaves already had a checkout when we 
switched to VS 2008, so a fresh checkout was never tested.

That might also explain the buildbot crashes - they still were using
object files from VS 2003, for db-4.4.20.

Regards,
Martin