[MLton] MinGW port

[Henry Cejtin]

With  regards  to  the  choice  of what to do about things that can't be done
under MinGW, I would think that choice A (remove  them  from  the  signature)
would  not  be compliant with the standard basis library spec.  It would mean
that some one might do an open expecting  something  to  be  masked  and  not
having that happen.

With  regards  to  the  choice  to not have any of the POSIX stuff work, that
seems bad to me because code that should port would not any more.

I think having the unsupported functions is the best we can do,  although  it
does  mean  that you don't know about the problem until run-time.  The best I
can imagine would be to warn at compilation time.