* rename Findrocksdb.cmake to FindRocksDB.cmake to match its name
* add RocksDB::RocksDB target to BuildRocksDB.cmake and
FindRocksDB.cmake
* use RocksDB::RocksDB target instead of accessing its property
directly, and do not link against its dependencies explicitly.
let its INTERFACE_LINK_LIBRARIES do the job.
Signed-off-by: Kefu Chai <kchai@redhat.com>
