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