I had the same problem too on slackware 13.1, here is my forum topic about it:
http://www.linuxquestions.org/questi...ackages-850734
I even tried to use directive LDFLAGS="-L/usr/lib${LIBDIRSUFFIX}", but it didn't help.
It looks like there is something wrong with gcc-g++ multilib package. Just try to install original gcc-g++ package from slackware64 before compiling MySql workbench, then after compilation is done you can install back multilib package, Workbench itself will work without problems, it is only about compiling.