Support setting spatialite module name via options #539
will not even try to visualize if PDAL_HAVE_PCL_VISUALIZATION not present
oops, still need the guard for now
check if visualization requested in Sort and Random as well
Clean up zlib's cmake logic

Previously, we still only linked against zlib if the cmake variable
WITH_ZLIB was true, but now that zlib is a required dependency, we
should always link against its libraries.

Additionally, use the REQUIRED flag for the zlib find_package.
Merge pull request #546 from gadomski/cmake-zlib

Clean up cmake's zlib logic
Merge pull request #539 from PDAL/issues/539-spatialite-module-name

Allow sqlite driver to specify spatialite module name
Merge pull request #538 from PDAL/remove-cannot-visualize-error

will not even try to visualize if PDAL_HAVE_PCL_VISUALIZATION not present