This is most likely to your tcl-library incorrectly being detected as non-threaded. One solution seem to be to manually build and install libtcl rather than use rpm/deb/etc packages.
There are a few other threads about this, such as this one
Using --enable-threads when compiling eggdrop will not help, as the configure-script will try to figure it out itself, and basically ignore your option :/
Manually installing tcl (threaded or non-threaded) has proven successful in solving this issue before.