/usr/include/gtkglextmm-1.2/gdkmm/gl/ is in libgtkglextmm-x11-1.2-dev 1.2.0-4.1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gtkglextmm-1.2/gdkmm/gl/config.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/context.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/defs.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/drawable.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/font.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/init.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/pixmap.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/pixmapext.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/config_p.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/context_p.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/drawable_p.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/pixmap_p.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/tokens_p.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/private/window_p.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/query.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/tokens.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/version.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/window.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/windowext.h
/usr/include/gtkglextmm-1.2/gdkmm/gl/wrap_init.h