Begin using 'option' rather than 'set' for PDAL options

Options become slightly more concise (drops the 'CACHE BOOL') and
enables use of CMAKE_DEPENDENT_OPTION macro, which could be useful
for the GDAL/GeoTIFF/TIFF trio among others.
small cleanups to filters.tiler
use make generator by default
Merge remote-tracking branch 'origin/use-options-command'