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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