GtkGLExt is an OpenGL extension to GTK+. It provides additional GDK objects which support OpenGL rendering in GTK+, and GtkWidget API add-ons to make GTK+ widgets OpenGL-capable.
Verson 1.2.0 generates a buggy library when compiled with GTK+2.x. Some symbols, like GTK_WIDGET_REALIZED, will cause linking to fail. The compile of "make examples" will fail. This bug has been getting fixed in Linux distributions since 2010. If you download 1.2.0 from here you will be fixing it yourself.