Bugs/Tcl8.6
From Eggdrop Wiki

./configure your bot. Open the config.h file in your editor.
Search for (around line 244)
/* Define for Tcl that has threads. */ /* #undef HAVE_TCL_THREADS */
and insert the following right after it
#define USE_INTERP_RESULT 1
Carry on with your make config && make && make install.