add WITH_FLANN guard so FLANN is no longer a required dependency
add WITH_FLANN guard so FLANN is no longer a required dependency

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