FLTK is described as 'The Fast, Light Toolkit (FLTK, pronounced fulltick) is a cross-platform graphical user interface (GUI) library developed by Bill Spitzak and others. Made to accommodate 3D graphics programming, it has an interface to OpenGL, but it is also suitable for general GUI programming' and is an app in the development category. There are more than 10 alternatives to FLTK for a variety of platforms, including Windows, Linux, Mac, BSD and Android apps. The best FLTK alternative is Qt, which is both free and Open Source. Other great apps like FLTK are Qt Creator, GTK, wxWidgets and PyQt.
QT Creator is an ide not a library