[tlbuild] dvibook failure on Cygwin

[Karl Berry]

    Ideally c-unistd.h could do something similar for Cygwin:

If I understand correctly, changing existing code to use
kpathsea/c-unistd.h rather than <unistd.h> doesn't seem "ideal" to me.
Just the opposite.

    I prefer the first alternative, since it's cleaner conceptually.  

For myself, the two ideas of "cleanliness" and "adapting to system
headers" do not go together at all :).  All we can do is what gets the
job done.  If there is a plausible way at all.  Or live with the
warnings on Cygwin :(.

karl