VS2013 is unsupported since https://github.com/google/benchmark/pull/691 / https://github.com/google/benchmark/commit/eb8cbec0776455463274ea9947ab0ecfe0f768fe
but i forgot to update docs.
References:
* https://github.com/google/benchmark/issues/689
* https://github.com/google/benchmark/pull/691
* https://github.com/google/googletest/pull/1815
* https://github.com/google/benchmark/issues/853
* https://github.com/google/benchmark/pull/854