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The example should be build with grpc++, not grpc++_unsecure.
For some reason in some branches like master, both versions are built in
so problem not detected. This problem surfaced in 1.28x so needs to be
fixed to link with the correct grpc++.
The Google Benchmark CMake build needs CMake 3.5.1 or newer. CMake 3.5.1
was released May 24, 2016 and is available in Debian stable and Ubuntu
16.04 and 18.04.