The license used for GLFW is known as the zlib/libpng license. It is very liberal, basically letting you do anything you want with the software (except for stealing it or miscrediting the authors). You will find the license in the GLFW distribution and in all the related source code files.
Here is how the license reads:
Copyright © 2002-2006 Camilla Berglund
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.