fplan was developed primarily for use on systems running the UNIX Operating System and the X11 Window System (with particular emphasis on Linux). If you want to build from source code on a UNIX system, you will need either the XView Toolkit, or the Gnome, Gtk+, Glib libraries to use the graphical previewer. On OS/2 and MS-DOS systems, the code can be built with the graphical previewer disabled.
It's available right now and you probably already have it! There is a common misconception among some Linux users that X11 applications will work correctly only under a specific desktop or X11 window manager such as Gnome or KDE. For the vast majority of applications this is simply not true. For example, consider an application that uses the OSF/Motif display widgets, the popular Netscape Navigator web browser is a perfect example. As you probably know from experience, you don't need to be running mwm, the Motif Window Manager, to use it. It runs fine under your favorite desktop or window manager; Afterstep, Gnome, KDE, fvwm, icewm, twm, olwm, etc. The situation with fplan is no different. It currently uses only the display widgets from Gnome and/or Xview, so it behaves the same under KDE as it does under any other desktop, window manager.
If you want to build fplan from source, you will need to have the Gnome and/or Xview libraries installed on your system, no matter what desktop you use. For the binary distributions of fplan, you don't need the libraries. The executables are statically linked for your convenience.