Do not ignore non-existent variables
Do not ignore non-existent variables

Fix build issue reported on IRC by Massimo Di Stefano
mtCarto/PDAL,mtCarto/PDAL,verma/PDAL,radiantbluetechnologies/PDAL,boundlessgeo/PDAL,mpgerlek/PDAL-old,jwomeara/PDAL,boundlessgeo/PDAL,lucadelu/PDAL,mpgerlek/PDAL-old,mtCarto/PDAL,DougFirErickson/PDAL,boundlessgeo/PDAL,verma/PDAL,mtCarto/PDAL,verma/PDAL,DougFirErickson/PDAL,lucadelu/PDAL,jwomeara/PDAL,Sciumo/PDAL,lucadelu/PDAL,verma/PDAL,mpgerlek/PDAL-old,DougFirErickson/PDAL,lucadelu/PDAL,boundlessgeo/PDAL,mpgerlek/PDAL-old,verma/PDAL,jwomeara/PDAL,verma/PDAL,jwomeara/PDAL,verma/PDAL,Sciumo/PDAL,radiantbluetechnologies/PDAL,DougFirErickson/PDAL,Sciumo/PDAL,radiantbluetechnologies/PDAL,radiantbluetechnologies/PDAL,Sciumo/PDAL