Depending on 1.6 is not sufficient as it does not contain HWLOC_OBJ_OSDEV_COPROC. Build fails with hwloc 1.6.2 but succeeds with 1.9.
The openmpi-1.8 sources suggest it uses hwloc 1.7.2 "internally".
Fixed for 1.7.5 and 1.8 as per OpenMPI Changelog, http://svn.open-mpi.org/svn/ompi/branches/v1.8/NEWS