You can find more information about the library on the website:

http://www.fixscript.org/

To use in your project, simply copy these files to your project:

fixgui.c
fixgui.h
fixgui_common.h
fixgui_win32.c
fixgui_cocoa.c
macros.fix
classes.fix
image/
gui/

Compile fiximage.c with these flags:
-msse2 -mstackrealign

On Windows you need to link against these libraries:
-lgdi32 -lcomdlg32 -lshlwapi -lcomctl32 -lwinmm -mwindows

On Mac OS X you need to link against these libraries:
-lm -lobjc -lpthread -framework Cocoa
