Use prefix for lightstep-tracer-cpp build so that protobuf tools will be found.
Use prefix for lightstep-tracer-cpp build so that protobuf tools will be found.

lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp