CMake: use HAVE_* rather than *_FOUND for plugins

Change-Id: I7a82c23af5dffe34e809fbd841eaef09dab54060
Reviewed-on: http://review.couchbase.org/33334
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>