Build fltk from source
$ ./configure --enable-xft --enable-threads --enable-x11 --enable-shared
The options on the configure script are essential. One source of information mentions them only as a footnote that can be overlooked. fltk will compile without error but fldigi will fail on make if these options aren’t set.
Another minor gotcha was that a second source had specified, “–enable-X11” instead of “–enable-x11”. Linux is case sensitive and “configure” will fail with a switch not recognised error.