Thanks, problem is solved. I thought the problem was in the linker, but silly me. It was the code. Thanks a lot.

i installed using homebrew to make the program on x code, but i faced the same linking issue there so i installed fltk 1.4 separately and tried to make the program using Make. I first tried to make my program using cmake and x code as described here : https://github.com/fltk/fltk/blob/master/README.macOS.md#build_cmake_make, but after completing all the steps up till building fltk, i couldn't figure out what to do with the CMakeLists.txt file or how to link my new project with fltk. After that i tried Homebrew and was able to successfully link my program with fltk but then this linking issue occured and i got stuck. Now, even with updating the code using the github page using you mentioned, i am getting the same issue. Can you tell me in which directory should i be in when i execute this command, if i installed fltk using Homebrew? What was the procedure you followed if you also did this exercise?