/usr/include/opengm/inference/external/ is in libopengm-dev 2.3.6+20160905-1build2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/opengm/inference/external/ad3.hxx
/usr/include/opengm/inference/external/daoopt.hxx
/usr/include/opengm/inference/external/fastPD.hxx
/usr/include/opengm/inference/external/gco.hxx
/usr/include/opengm/inference/external/grante.hxx
/usr/include/opengm/inference/external/libdai/
/usr/include/opengm/inference/external/libdai/bp.hxx
/usr/include/opengm/inference/external/libdai/dec_map.hxx
/usr/include/opengm/inference/external/libdai/double_loop_generalized_bp.hxx
/usr/include/opengm/inference/external/libdai/exact.hxx
/usr/include/opengm/inference/external/libdai/fractional_bp.hxx
/usr/include/opengm/inference/external/libdai/gibbs.hxx
/usr/include/opengm/inference/external/libdai/inference.hxx
/usr/include/opengm/inference/external/libdai/junction_tree.hxx
/usr/include/opengm/inference/external/libdai/loop_corrected_bp.hxx
/usr/include/opengm/inference/external/libdai/mean_field.hxx
/usr/include/opengm/inference/external/libdai/tree_expectation_propagation.hxx
/usr/include/opengm/inference/external/libdai/tree_reweighted_bp.hxx
/usr/include/opengm/inference/external/mplp.hxx
/usr/include/opengm/inference/external/mrflib.hxx
/usr/include/opengm/inference/external/qpbo.hxx
/usr/include/opengm/inference/external/srmp.hxx
/usr/include/opengm/inference/external/trws.hxx
/usr/include/opengm/inference/external/typeView.h