The libSBOLj library is libSBOL's Java implementation, whereas libSBOLc is a preliminary C implementation of libSBOL. Both libraries are under continuous development. The sources are licensed using the Apache 2.0 open source license, and are freely available at the SynBioDex site on Github.
If you would like to hack on the libSBOLj source, please read our github guide. It should cover everything you need to get started or to contribute back to the project. The code is built using maven. If you are unfamiliar with it, see our maven guide.
For further information about the libSBOL library, its implementation, or its usage, please feel free to contact the libSBOL developers.