Allow custom lib targets to be specified in the generator_config

[scotthart]

99% of the time, the lib path for a service is simply "./lib64/cmake/google_cloud_cpp_" + lib. For the other 1% we need to specify a custom path to emit into ci/etc/expected_install_directories. We can either hard code these few exceptions or add support to generically specify them.

[dbolduc]

This is less relevant, as we generated the BigQuery v2 API as its own bigquerycontrol library.