Fltk install on Linux

Download FreeBASIC from: https://sourceforge.net/projects/fbc/files/
(select the version for your OS/architecture)
Download Fltk from: http://shiny3d.de/public/libs/fltk-c/fltk-c-1.3.3.zip

Extract the FreeBASIC file to your preferred folder.
Then start a terminal and login in as super user (su) or use sudo.
# cd <path to your freebasic folder>
# chmod a+x install.sh
# ./install.sh

You can find some basic instructions for Fltk installation on:
https://www.freebasic.net/forum/viewtopic.php?f=14&t=24547&hilit=fltk
In most cases this works.

With other distributions like antiX you will run in problems.
Download the Fltk library and add the library in the local FreeBASIC folder like in the image below.

Screenshot fltk folder

Run the install.sh script again like this:
# chmod a+x install.sh
# ./install.sh

Then install the mesa development libraries for Gtk.
This will prevent getting the cannot find -lGL error.
# apt-get install libgl1-mesa-dev

After this your FreeBASIC Fltk projects will compile.