feat: support install components for the library (googleapis/google-cloud-cpp-spanner#659)
feat: support install components for the library (googleapis/google-cloud-cpp-spanner#659)

TIL: we can make it easier for package maintainers to create separate
packages for development vs. runtime if we use named components for
the library install directories. With CMake 3.5, however, we need to
use a secondary `install()` command for the libFoo.so.X.Y.Z link (aka
the NAMELINK_ONLY install)

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp