message: handle old boost
diff: #ifndef SPARSIFY_POINT_SET_H_
  #define SPARSIFY_POINT_SET_H_
  +#include <boost/version.hpp>
  +#if BOOST_VERSION < 106600
  +# include <boost/function_output_iterator.hpp>
  +#else
  # include <boost/iterator/function_output_iterator.hpp>
  +#endif
  #include <gudhi/Kd_tree_search.h>
  #ifdef GUDHI_SUBSAMPLING_PROFILING
  @@ -53,7 +58,6 @@ sparsify_point_set(
  OutputIterator output_it) {
  typedef typename Gudhi::spatial_searching::Kd_tree_search<
  Kernel, Point_range> Points_ds;
  - using std::sqrt;
  #ifdef GUDHI_SUBSAMPLING_PROFILING
  Gudhi::Clock t;
  