あと、ライセンスがzlibなのでいいですね(本家のFAQより)。
Can I use the engine in a non open source commercial product?
Yes you can. The engine is open source and totally free. You can debug it,
fix bugs and even change things you do not like.
And you do not have to publish your changes:
The engine is licensed under the zlib license,
not the GPL or the LGPL.
Look at license for more informations.