Aside from the posted library sj.h which is in public domain (compatible with the definition of "free software"), the author's other projects mostly use the MIT license.

The MIT license upholds the four essential freedoms of free software: the right to run, copy, distribute, study, change and improve the software.

It is listed under "Expat License" in the list of GPL-compatible Free Software licenses.

https://www.gnu.org/licenses/license-list.html