Upon installing eggdrop bot and have error, try this;

one of the reason why you can’t continue setting up your eggdrop..

CFLAGS="-std=gnu89" ./configure --with-tcllib='/usr/lib/x86_64-linux-gnu/libtcl8.6.so' --with-tclinc='/usr/include/tcl8.6/tcl.h'

this only apply for Eggdrop1.6.20.

checking whether the Tcl system has changed… yes
configure: error:

Tcl cannot be found on this system.

Eggdrop requires Tcl and the Tcl development files to compile.
If you already have Tcl installed on this system, make sure you
also have the development files (common package names include
'tcl-dev' and 'tcl-devel'). If I just wasn't looking
in the right place for it, re-run ./configure using the
--with-tcllib='/path/to/libtcl.so' and
--with-tclinc='/path/to/tcl.h' options.

See doc/COMPILE-GUIDE's 'Tcl Detection and Installation' section for more
information.

Try to configure like this:

./configure --with-tclinc=/usr/include/tcl8.6/tcl.h --with-tcllib=/usr/lib/x86_64-linux-gnu/libtcl8.6.so

if that doesn`t help, than you only need to install build essential and tcl-dev:

apt-get install build-essential
apt-get install tcl-devel

If that doesn`t works, try this as your last chance: instead of ./configure, use this line:

CFLAGS="-std=gnu89" ./configure --with-tclinc=/usr/include/tcl8.6/tcl.h --with-tcllib=/usr/lib/x86_64-linux-gnu/libtcl8.6.so

If these commands didn`t help, than try to understand what are you doing so you never face the same issues again.