Without this change, new users get this warning:
```
CMake Warning (dev) at $MY_INSTALL_DIR/lib64/cmake/protobuf/protobuf-options.cmake:6 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.
For compatibility with older versions of CMake, option is clearing the
normal variable 'protobuf_MODULE_COMPATIBLE'.
Call Stack (most recent call first):
$MY_INSTALL_DIR/lib64/cmake/protobuf/protobuf-config.cmake:2 (include)
$MY_SRC_PATH/examples/cpp/cmake/common.cmake:99 (find_package)
CMakeLists.txt:24 (include)
This warning is for project developers. Use -Wno-dev to suppress it.
```
release notes: no
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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.