(See also
http://castle-engine.sourceforge.net/engine.php#section_license)

The core of "Castle Game Engine" (almost everything that can be found
in this directory, castle_game_engine) may be used on the terms of:

1. GNU General Public License (version 2 or above, whichever you like),
   see COPYING.GPL2.txt for GPL 2 text,
   see http://www.gnu.org/copyleft/gpl.html for current version, FAQs etc.

2. Or on a more permissive
   GNU Lesser General Public License (again version 2 or above, whichever you like),
   (also called "Library General Public License" in earlier versions),
   see COPYING.LGPL2.txt for LGPL 2 text,
   see http://www.gnu.org/copyleft/lesser.html for current version, FAQs etc.

   Please note that if you want to use LGPL:

   - You *have* to modify src/base/castleconf.inc
     to make symbol CASTLE_ENGINE_LGPL defined. This will avoid pulling
     any GPL-only (strict) dependencies into the engine --- currently
     this concerns NURBS implementatiopn.

   - Also, we add the so-called "static linking exception" to LGPL.
     The idea is that you can statically link closed-source code
     with the engine, the exact legal text follows:

     As a special exception, the copyright holders of this library
     give you permission to link this library with independent
     modules to produce an executable, regardless of the license
     terms of these independent modules, and to copy and distribute
     the resulting executable under terms of your choice,
     provided that you also meet, for each linked independent module,
     the terms and conditions of the license of that module.
     An independent module is a module which is not derived from
     or based on this library. If you modify this library,
     you may extend this exception to your version of the library,
     but you are not obligated to do so. If you do not wish to do so,
     delete this exception statement from your version.