Summary: Rewrite `java/CMakeLists.txt` to take advantage of CMake's cross platform Java support. adamretter Closes https://github.com/facebook/rocksdb/pull/2301 Differential Revision: D5070724 Pulled By: sagar0 fbshipit-source-id: 999aee9bd39da2b24a5fe493a2eb0e9af6072dc7