[tlbuild] dvibook failure on Cygwin

[Karl Berry]

    I don't understand why we need unistd.h (and why only on Cygwin) 

Of course I can't guess what Cygwin is doing in the system headers, but
I can't say I'm surprised.  The "dual nature" of kpathsea/getopt.h,
being both kpathsea-specific but also a system library routine, is
clearly a source of potential confusion, and in the case of Cygwin, the
potential has been realized.

I suggest we just put in the #include and get on with life.

Best,
Karl