Summary:
- cmake: use the builtin FindBzip2.cmake from CMake
- cmake: require CMake v3.5.1
- cmake: add imported target for 3rd party libraries
- cmake: extract ReadVersion.cmake out and refactor it
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5662

Differential Revision: D16660974

Pulled By: maysamyabandeh

fbshipit-source-id: 681594910e74253251fe14ad0befc41a4d0f4fd4