graphics/luxrender14: unbreak with boost 1.68

accelerators/bvhaccel.cpp:149:19: error: call to 'distance' is ambiguous

u_int middle = distance(list.begin(), it);
               ^~~~~~~~

/usr/include/c++/v1/iterator:511:1: note: candidate function [with _InputIter = std::1::wrap_iter<boost::shared_ptr<lux::BVHAccelTreeNode> *>]
distance(_InputIter first, _InputIter last)
^
/usr/local/include/boost/iterator/distance.hpp:49:9: note: candidate function [with SinglePassIterator = std::1::wrap_iter<boost::shared_ptr<lux::BVHAccelTreeNode> *>]

distance(SinglePassIterator first, SinglePassIterator last)
^

PR: 229569
Reported by: antoine (via exp-run)