This directory is designated to keep "external" libraries used by some of the extensions. "External" means any non-Qt, non-CuteHMI libraries. They are either supplementing Qt with their own functionality, or they are required by some of the Qt plugins (e.g., SQL drivers).
By default only Make recipes are kept in the repository. To build the libraries one has to evaluate recipes with a suitable toolchain. Check out recipes directory for further details.