Allow build on Boost 1.56
Allow build on Boost 1.56

mpgerlek/PDAL-old,jwomeara/PDAL,jwomeara/PDAL,mpgerlek/PDAL-old,lucadelu/PDAL,mpgerlek/PDAL-old,Sciumo/PDAL,DougFirErickson/PDAL,Sciumo/PDAL,lucadelu/PDAL,lucadelu/PDAL,DougFirErickson/PDAL,radiantbluetechnologies/PDAL,Sciumo/PDAL,boundlessgeo/PDAL,mtCarto/PDAL,jwomeara/PDAL,boundlessgeo/PDAL,lucadelu/PDAL,Sciumo/PDAL,radiantbluetechnologies/PDAL,mtCarto/PDAL,DougFirErickson/PDAL,DougFirErickson/PDAL,jwomeara/PDAL,boundlessgeo/PDAL,mtCarto/PDAL,mtCarto/PDAL,radiantbluetechnologies/PDAL,radiantbluetechnologies/PDAL,boundlessgeo/PDAL,mpgerlek/PDAL-old