Enable building the CubeView demo w/o requiring '<config.h>'

This allows to build the CubeView demo from the unmodified FLTK sources
w/o having to provide a 'config.h' file by defining HAVE_GL=1 on the
compiler commandline. See also comment in test/CubeView.h.

test/CubeMain.cxx: don't include <config.h> at all (not needed)

test/CubeView.h: #include <config.h> only if HAVE_GL is not defined

test/CubeMain.cxx

@@ -1,7 +1,7 @@
 //
 // CubeView class .
 //
-// Copyright 1998-2010 by Bill Spitzak and others.
+// Copyright 1998-2024 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -14,7 +14,6 @@
 //     https://www.fltk.org/bugs.php
 //
 
-#include <config.h>
 #include <FL/Fl.H>
 #include "CubeViewUI.h"
 

test/CubeView.h

@@ -1,7 +1,7 @@
 //
 // CubeView class definitions for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2021 by Bill Spitzak and others.
+// Copyright 1998-2024 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -17,7 +17,16 @@
 #ifndef CUBEVIEW_H
 #define CUBEVIEW_H 1
 
+// Note: The CubeView demo program can only be built if
+//  (1) '<config.h>' can be included (FLTK build) *or*
+//  (2) 'HAVE_GL' has been defined prior to including this header.
+
+// Define 'HAVE_GL=1' on the compiler commandline to build
+// this program w/o 'config.h' (needs FLTK lib with GL).
+
+#ifndef HAVE_GL
 #include <config.h>
+#endif
 
 // Note to editor: the following code can and should be copied
 // to the fluid tutorial in 'documentation/src/fluid.dox'