I just built graphviz-2.26.2.tar.gz using the current Slackbuild from SBo on a clean Slackware 13, pure x86_64 which I also just finished installing - no problems here.
Maybe you should reinstall your zlib package in case it was corrupted?
Also check the MD5 on the source maybe...
EDIT: HA! I see you got it going while I was compiling! Good luck! /EDIT:
Last edited by astrogeek; 03-14-2010 at 10:32 PM.