reflecting differences between Linux and Cygwin in basis library

[Stephen Weeks]

> In the specific example you give, I would provide  a  Cygwin  .h  file  which
> defines the constant and a new C routine which checks for the new bogus value
> and raises an exception.  
...
> I would definitely not go the intersection route in general.

I am OK with this direction, but since we are talking about what is
exported to SML programmers and not something that is needed for the
runtime system implementation, I would rather put the code to raise
the exception in the SML basis library implementation than in the C
code, on general principles (more exposed to optimizer, easier to
write bug free code, etc.).