This license is based on zlib, but adds an additional clause which is intended to prevent sublicensing. While this clause does not make this license non-free, it does make it GPL-incompatible. A copy of this license was taken from: http://src.gnu-darwin.org/ports/games/assaultcube/work/AssaultCube/source/LICENSE
Cube game engine source code, 20 dec 2003 release. Copyright (C) 2001-2003 Wouter van Oortmerssen. This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software. Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions: 1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required. 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software. 3. This notice may not be removed or altered from any source distribution. additional clause specific to Cube: 4. Source versions may not be "relicensed" under a different license without my explicitly written permission.