Some changes needed to compile on 64bit.
1. size_t is not int - the best way to print size_t is by casting to (unsigned long) and using %lu in the string format.
2. Printing the buffer without format string is considered a security warning so instead of ZPrint(buffer) you should use ZPrint(%s, buffer)
3. (u_char) NULL ---> (u_char) 0
etc.
I can provide the patches if you're interested.
Is the project still alive?
Thanks,
Slavik