If `-Dprotobuf_USE_EXTERNAL_GTEST=ON` is passed to CMake, it will use an external Google Test copy (i.e. by calling `find_package(GTest REQUIRED)`) rather than the one provided as a submodule.
This makes sense for larger projects that might already include Google Test and want to use a more standard CMake approach.
Also updated build instructions with this information, and more idiomatic usage.
Co-authored-by: Adam Cozzette <acozzette@google.com>
Convenience feature: enable users to test via the familiar `ctest` command rather than making the `check` target.
They would be able to use the familiar CMake pattern:
```
cmake -S source/protobuf -B build/protobuf ...
cmake --build build/protobuf
ctest --test-dir build/protobuf
cmake --install build/protobuf
```
This is a follow-up to 9f447fc9d3da93da29b8301f1a8ca57b1ea812d7
