/usr/include/ is in libgtkglext1-dev 1.2.0-4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gtkglext-1.0/gtk/
/usr/include/gtkglext-1.0/
/usr/include/gtkglext-1.0/gdk/
/usr/include/gtkglext-1.0/gdk/gdkgl.h
/usr/include/gtkglext-1.0/gdk/gdkglconfig.h
/usr/include/gtkglext-1.0/gdk/gdkglcontext.h
/usr/include/gtkglext-1.0/gdk/gdkgldebug.h
/usr/include/gtkglext-1.0/gdk/gdkgldefs.h
/usr/include/gtkglext-1.0/gdk/gdkgldrawable.h
/usr/include/gtkglext-1.0/gdk/gdkglenumtypes.h
/usr/include/gtkglext-1.0/gdk/gdkglfont.h
/usr/include/gtkglext-1.0/gdk/gdkglglext.h
/usr/include/gtkglext-1.0/gdk/gdkglinit.h
/usr/include/gtkglext-1.0/gdk/gdkglpixmap.h
/usr/include/gtkglext-1.0/gdk/gdkglquery.h
/usr/include/gtkglext-1.0/gdk/gdkglshapes.h
/usr/include/gtkglext-1.0/gdk/gdkgltokens.h
/usr/include/gtkglext-1.0/gdk/gdkgltypes.h
/usr/include/gtkglext-1.0/gdk/gdkglversion.h
/usr/include/gtkglext-1.0/gdk/gdkglwindow.h
/usr/include/gtkglext-1.0/gdk/glext/
/usr/include/gtkglext-1.0/gdk/glext/glext-extra.h
/usr/include/gtkglext-1.0/gdk/glext/glext.h
/usr/include/gtkglext-1.0/gdk/glext/glxext-extra.h
/usr/include/gtkglext-1.0/gdk/glext/glxext.h
/usr/include/gtkglext-1.0/gdk/glext/wglext-extra.h
/usr/include/gtkglext-1.0/gdk/glext/wglext.h
/usr/include/gtkglext-1.0/gdk/x11/
/usr/include/gtkglext-1.0/gdk/x11/gdkglglxext.h
/usr/include/gtkglext-1.0/gdk/x11/gdkglx.h
/usr/include/gtkglext-1.0/gtk/gtkgl.h
/usr/include/gtkglext-1.0/gtk/gtkgldebug.h
/usr/include/gtkglext-1.0/gtk/gtkgldefs.h
/usr/include/gtkglext-1.0/gtk/gtkglinit.h
/usr/include/gtkglext-1.0/gtk/gtkglversion.h
/usr/include/gtkglext-1.0/gtk/gtkglwidget.h