Needed: Automatic detection of dependencies on Windows

As #690 (closed) shows again, the dependencies of some externals we ship on Windows keep changing. A proper solution is in order, along the lines of the dependencies script that we have on the Mac, which takes care of at least some of the most important and problematic externals, in particular Gem and fluid~. (I think that a full-blown dependencies script, like the one we use on the Mac, would be overkill for now, and in any case it would probably be quite slow on the Windows file system.)