I tried building the program from source on Gentoo but it fails due to fltk library not found. The thing is fltk is there, but in the fltk-1 subdirectory of libdir so it is not found. Assuming fltk is there if fltk-config should be safe and thus that is the approach I'm suggesting. I'm submitting a patch fixing the issue.
I also fixed two typos in configure.ac and cleaned up a little in src/Makefile.