Update leveldb to version 1.12.0. Major changes:

- Fix corruption bug found and analyzed by dhruba@gmail.com
  https://groups.google.com/d/msg/leveldb/Kc9JxuIUu5A/9P0N9RL4ar8J

- Make DB::Open fail if sst files are missing.
  Also, cleanup for Clang's -Wimplicit-fallthrough warning.

- Fixes issues 147 153 156 166
  Remove calls to exit(1).
  Fix unused-variable warnings from clang.
  Fix possible overflow error related to num_restart value >= (2^32/4).
  Add leveldbutil to .gitignore.
  Add better log messages when Write is stalled on a compaction.

- Fixes issues 161 174 178
  As well as the issue reported by edouarda14@gmail.com about
  MissingSSTFile unit test failing on windows.

- Release leveldb 1.12
  Non-functional changes only:
  1) Add Kevin Regan to the AUTHORS file
  2) Add standard license header to issues/issue178_test.cc

In addition, the package now autodetects library version.