/usr/include/allegro5/opengl/GLext/ is in liballegro5-dev 2:5.2.3.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/allegro5/opengl/GLext/gl_ext_alias.h
/usr/include/allegro5/opengl/GLext/gl_ext_api.h
/usr/include/allegro5/opengl/GLext/gl_ext_defs.h
/usr/include/allegro5/opengl/GLext/gl_ext_list.h
/usr/include/allegro5/opengl/GLext/glx_ext_alias.h
/usr/include/allegro5/opengl/GLext/glx_ext_api.h
/usr/include/allegro5/opengl/GLext/glx_ext_defs.h
/usr/include/allegro5/opengl/GLext/glx_ext_list.h
/usr/include/allegro5/opengl/GLext/wgl_ext_alias.h
/usr/include/allegro5/opengl/GLext/wgl_ext_api.h
/usr/include/allegro5/opengl/GLext/wgl_ext_defs.h
/usr/include/allegro5/opengl/GLext/wgl_ext_list.h