Getting the same error when trying to compile some KDE apps based on a Slack9.0 install of qt-3.1.2. According to many things I've read, Qt3 compiles with -thread in by default, though it can't hurt to specify it.
If you're going to be removing it, you'd just remove your entire $QTDIR directory tree, and recompile with the -thread option passed to ./configure.
I'm not looking forward to having to do this, myself. libqt-mt is there, and I still get threading errors. Looks like I may have to remove the Slack package and compile from source.
Trying to compile from source with the Slack package in place throws errors about 10 minutes into the compile.