Ah OK. Then you need to do this when you generate the .c files:
gsc -c -o glut-ffi.o1.c glut-ffi.scm gsc glut-ffi.o1.c
This will generate the files glut-ffi.o1.c and glut-ffi.o1_.c which you can then compile as before.
Scratch that... since you are generating an object file, all you need is
gsc -c -o glut-ffi.o1.c glut-ffi.scm $(CC) -shared -D___DYNAMIC -I/usr/include/GL -I $(GAMBINC) -o glut-ffi.o1 glut-ffi.o1.c conv.o -L/usr/X11R6/lib/ -lgl
Marc