Fix the build break on Ubuntu 15.10 when gcc 5.2.1 is used
Fix the build break on Ubuntu 15.10 when gcc 5.2.1 is used

Summary:
Using gcc 5.2.1 to compile RocksDB on Ubuntu 15.10 results in a warning about unused variable. Warning is treated as an error and all of it results in a build break. Fix the issue and make sure that everything compiles with that particular configuration.

P.S. Lint complains about a non-ASCII character used in the source code ("Wagner-Fischer"). Fix this as well.

Test Plan:
  # Everything compiler cleanly on the system which exhibited the problem.
  # `make clean && make -j 16` on CentOS.
  # `make clean && USE_CLANG=1 make -j 16` on CentOS.

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D55503

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