GiGi (aka GG) is a C++ OpenGL GUI library.  It has drivers that work with SDL
(https://www.libsdl.org) and Ogre (https://www.ogre3d.org).

For documentation of building GG, you can check out:

https://gigi.sourceforge.net/doxygen

However, this documentation is often pretty out of date, so try building the
documentation from the sources.

For information on building GG from sources, see INSTALLING.
For information on packaging GG for Linux distributions, see PACKAGING.
For the license used by GG, see COPYING.

Enjoy!

Zach Laine