-lfltk
is not enough to build a minimal FLTK-based application.
I strongly suggest you to read the following page: http://www.fltk.org/doc-1.3/basics.html . It clearly explains how to compile a simple FLTK program.
Assuming your FLTK application is not using extra FLTK libraries (opengl, images, forms) and is in a single file, called myfltkapp.cpp, the absolutely quickest way to compile and link it properly would be:
fltk-config --compile myfltkapp.cpp