Port remainder of drivers with third-party dependencies to plugin framework

Ported plugins include:

* HexBin
* Icebridge
* MrSID
* NITF
* OCI
* Points2Grid
* PCL
* PgPointCloud
* SQLite

Begin using BUILD_PLUGIN_<NAME> to control optional building of plugins. Reserve use of WITH_<NAME> to control inclusion of an optional dependencies.

@gadomski If this PR is merged, PDAL_ADD_PLUGIN changes a bit, forming the library name internally within the macro, and accepting a final argument for any dependencies of the plugin. The target_link_libraries on line 21 goes away, and line 20 of your RXP CMakeLists  in #567 would become:

    PDAL_ADD_PLUGIN(rxp_reader_lib_name reader rxp "${srcs}" "${incs}" ${RiVLib_SCANLIB_LIBRARY})