Print a suggestion for a common Windows footgun

[mtzguido]

After a wild ride trying to figure out what was failing here (snippet below)

   CHECK           Steel.Semantics.Hoare.MST.fst
* Error 151:
  - Not a valid FStar file: '..'

@tahina-pro reminded me about the crazy expansion of wildcards in Windows. The shell does not expand them, the programs do it instead. So when we pass --already_cached '*' to F*, that asterisk is expanded to a list of arguments even if it's quoted. AFAIK this is done by some code injected into the program by the MinGW compiler, which we're using for Windows, and I'm not sure how to avoid it. This is why some --already_cached had ',*' instead, a trick I forgot about in some new Makefiles.

This PR documents the trick, by detecting this particular error on Windows builds and warning the user. (The asterisk is expanded to . .., and then some files; the first . is taken to be an argument of --already_cached and we fail on the second.) Now we get this:

(We also discovered that non-MinGW Cygwin builds (the ones you get if you install cygwin, clone the repo and build) are completely broken. I will follow up on that.)