Summary: Adds support for CMake packages: https://cmake.org/cmake/help/v3.9/manual/cmake-packages.7.html#creating-packages. This allow using RocksDB by other CMake projects this way: ``` cmake_minimum_required(VERSION 3.5) project(rdbt) find_package(RocksDB CONFIG) add_executable(rdbt test.cpp) target_link_libraries(rdbt PRIVATE RocksDB::rocksdb) ``` Closes https://github.com/facebook/rocksdb/pull/2773 Differential Revision: D5722587 Pulled By: sagar0 fbshipit-source-id: 0d90dc4a77b42a617cdbe1348a370e719c282b87