FLTK/OpenGL dependency

[Michael Goffioul]

I noticed that with the current octave code, compilation fails if
FLTK or OpenGL is not present. The problem is that the
configure script runs fine (some warning are simply reported),
but compilation fails, because it can't find FLTK or OpenGL
headers.

There 2 solutions:
1) make configure script to fail if FLTK/OpenGL is not present
2) make the code compile fine, even if FLTK/OpenGL is not present
  2a) deal with that in src/Makefile.in
  2b) deal with that in source code with #ifdef's

What should we do?
(I tend to think 2a is probably the best)

Michael.