Unexpected linker warnings on macOS builds

The macOS + CMake builds are issuing warnings such as:

```
ld: warning: ignoring duplicate libraries: 'google/cloud/bigtable/libgoogle_cloud_cpp_bigtable.a', 'google/cloud/bigtable/libgoogle_cloud_cpp_bigtable_protos.a', 'google/cloud/libgoogle_cloud_cpp_common.a', 'google/cloud/libgoogle_cloud_cpp_grpc_utils.a', 'vcpkg_installed/arm64-osx/debug/lib/libgmock.a', 'vcpkg_installed/arm64-osx/debug/lib/libgrpc++.a', 'vcpkg_installed/arm64-osx/debug/lib/libgrpc.a', 'vcpkg_installed/arm64-osx/debug/lib/libgtest.a', 'vcpkg_installed/arm64-osx/debug/lib/libprotobufd.a', 'vcpkg_installed/arm64-osx/debug/lib/manual-link/libgmock_main.a'
```

https://github.com/googleapis/google-cloud-cpp/actions/runs/7934388869/job/21665281291

These look new to me, and are probably not a great experience.

I remember seeing similar warnings with macOS + Bazel:

```
ld: warning: ignoring duplicate libraries: '-lc++'
```

https://github.com/googleapis/google-cloud-cpp/actions/runs/7926236215/job/21640717801

This suggests the toolchain may be issuing new warnings?