fltk

1. Setup

1) apt-get libfltk1.1-dev

2) compile:
fltk-config --compile

 g++  -I/usr/local/include -L/usr/local/lib -lX11 -ljpeg -lpng face_detector.cpp -lfltk_images -lfltk -o face_detector

3) to compile fltk loading images

2. development
when you try to compile the source code
you might bump into some problem cannot compile
bcs of lack of suitable library for developing
to solve this:
sudo apt-get build-dep fltk1.1
sudo apt-get source -b fltk1.1
http://ubuntuforums.org/showthread.php?t=86239
then go to the src code file
make clean
autoconf
./configure
make

3. reference 

official doc
good tutorial
forum