[MLton] mingwows

[Matthew Fluet]

> I understand this was fixed in 3.5 but  I could be wrong. It would be nice to 
> have a POSIX environment for MLton without the need for Cygwin. Actually, we 
> don't really need a POSIX environment, just a good implementation of the 
> Posix structure for the Basis. So in a pure mingwin env what's missing from 
> the POSIX structure?

It's not just the Posix structure in the Basis; much of the runtime and 
much of the OS.* structures in the Basis depend on Posix.