benchmark

Add a `benchmark_deps.bzl` function to Google Benchmark (#1520)

* Add `benchmark_workspace.bzl` function

This commit adds a `benchmark_workspace.bzl` function to Google
Benchmark. It is intended to be used as a means to pull in Google
Benchmark's build dependencies in its own Bazel workspace, as well
as in workspaces of downstream projects.

* Migrate WORKSPACE to use the newly created benchmark_deps.bzl

This commit changes Google Benchmark's own WORKSPACE to use the newly
created `benchmark_deps.bzl` function.