[tlbuild] dvibook failure on Cygwin

[Akira Kakuto]

Dear Ken,

>>    Something is seriously wrong with getopt
>>    (from kpathsea) on Cygwin.
>>
>> And always has been, and just never been noticed before?
>> Possible, I suppose, but doesn't seem especially likely.

Try by adding a prototype declaration in dvibook.c,
since stdlib.h seems not to be read:

int atoi (const char *str);

If ok, I see atoi() also in dviconcat.c and font.c.

Thanks,
Akira