The link to cmake webinars uner Resources in the project readme returns a custom 404 page. Unable to determine if the same content is being hosted elsewhere. Maybe remove link?
The osg-3rdparty-cmake uses a lot of licenses actually, since every dependency it builds have its own license. Hence all dependency specific build scripts must be considered as derivative works and thus must adhere to these licenses.
But the main script which holds everything together is written by me and is using the unlicense. http://unlicense.org/