fix leveldb missing Makefile

[gillesdemey]

LevelDB recently switched to CMake, effectively removing the existing Makefile from the master branch ref: https://github.com/google/leveldb/commit/8e75db8623703cdc25ec3cd06f82129296672489

fixes #56

[perlun]

Thanks for fixing @gillesdemey. But is this "the right way" to fix this? Shouldn't we perhaps be building leveldb using CMake instead?

[gillesdemey]

This is more of a work-around for the current code to build, but yes we should look into building LevelDB with CMake to keep up with its master branch.

[gillesdemey]

Closing this over #58