Disable pgpointcloud plugin by default

[gadomski]

I think users on a blank or close-to-blank system should see as few cmake errors as possible — since this was on by default, a fresh build on a mostly fresh system would throw an error.

[strk]

ideally, the default should be based on the availability of dependencies on the system, building it if possible, and not if not possible

[gadomski]

Something like: flip on the plugin if cmake's find_package(PostgreSQL) succeeds, but keep it off if it fails?

[strk]

Something like that, but I don't know what dependencies the pgpointcloud driver has exactly.

[gadomski]

I've updated the pull request to quietly search for PostgreSQL, and use POSTGRESQL_FOUND to initialize the option.

[strk]

works for me (builds it, as desired)