/usr/lib/x86_64-linux-gnu/ is in libgtkmm-3.0-dev 3.10.1-0ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/x86_64-linux-gnu/gdkmm-3.0/include/
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/m4/
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/m4/convert_gdk.m4
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/m4/convert_gtkmm.m4
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/m4/convert.m4
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/m4/convert_gtk.m4
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/proc/m4/class_gtkobject.m4
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/include/
/usr/lib/x86_64-linux-gnu/gtkmm-3.0/include/gtkmmconfig.h
/usr/lib/x86_64-linux-gnu/gdkmm-3.0/
/usr/lib/x86_64-linux-gnu/gdkmm-3.0/include/gdkmmconfig.h
/usr/lib/x86_64-linux-gnu/pkgconfig/
/usr/lib/x86_64-linux-gnu/pkgconfig/gdkmm-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtkmm-3.0.pc
/usr/lib/x86_64-linux-gnu/libgtkmm-3.0.so
/usr/lib/x86_64-linux-gnu/libgdkmm-3.0.so