Currently, hwloc's configuration.sh searched for any hwloc library and uses it. However, it requires a pretty new version within some of its own files, which leads to build failure on, e.g. Debian stable systems.
The attached patch uses pkg-config to get the version of the installed hwloc library and if that version is older than 1.6 (educated guess, but see http://lists.einsteintoolkit.org/pipermail/users/2013-February/002860.html), builds the bundled version even if another version is installed (but too old).
Note that this is done (intentionally) only if no library was specified, and the script was looking for it by itself. This allows users to specify something which will not be overwritten by this mechanism.