The zlib/libpng License
Copyright (c) 2015 Andrew Fenn Copyright (c) 2005-2010 Phillip Castaneda (pjcast -- www.wreckedgames.com)
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.
In case the license was not clear enough... Basically, you can link with this lib which puts no restrictions on what you have to license your code as. You can modify this lib, and not have release your changes. Though, as explained above, Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
This is to allow users the greatest flexibility in what you can use this lib for.