[MLton] MinGW port

[Stephen Weeks]

> For MinGW I think the right thing is to *not* rename
> the file, since all the executables are expected to
> have *.exe extensions.  Remember, MinGW is trying to
> produce real native Win32 executable, not UNIX-y stuff
> like Cygwin, so it *wants* to look like a windows app.

Fair enough.

> Summary:
> 
> 1.  Raise SysErr for any unimplemented code (in the
> MLton library).
> 2.  Leave any working POSIX stuff in place.
> 3.  Don't document that any POSIX layer exists for
> Win32 until it all works (or at least until a
> significant subset works).

OK.