/usr/lib/x86_64-linux-gnu/ is in libgtkmm-2.4-dev 1:2.24.4-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/x86_64-linux-gnu/pkgconfig/
/usr/lib/x86_64-linux-gnu/pkgconfig/gtkmm-2.4.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdkmm-2.4.pc
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/m4/
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/m4/convert_gtk.m4
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/m4/class_gtkobject.m4
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/m4/convert_gtkmm.m4
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/m4/convert_gdk.m4
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/proc/m4/convert.m4
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/include/
/usr/lib/x86_64-linux-gnu/gtkmm-2.4/include/gtkmmconfig.h
/usr/lib/x86_64-linux-gnu/gdkmm-2.4/
/usr/lib/x86_64-linux-gnu/gdkmm-2.4/include/
/usr/lib/x86_64-linux-gnu/gdkmm-2.4/include/gdkmmconfig.h
/usr/lib/x86_64-linux-gnu/libgtkmm-2.4.so
/usr/lib/x86_64-linux-gnu/libgdkmm-2.4.so