drake: have libprotoc's rpath use $ORIGIN
drake: have libprotoc's rpath use $ORIGIN

Currently we have builds on the BF that fail with:

    GRPC protoc src/memo/grpc/memo_vs_with_named.proto
    /cache/memo/release/0.9.0/src/memo/grpc/grpc/grpc_cpp_plugin: error while loading shared libraries: libprotoc.so: cannot open shared object file: No such file or directory
    --grpc_out: protoc-gen-grpc: Plugin failed with status code 127.
    Ran GRPCCxxGen in 0:00:00
    GRPCCxxGen: command failed: src/memo/grpc/grpc/third_party/protobuf/protoc -I src/memo/grpc --grpc_out=src/memo/grpc --plugin=protoc-gen-grpc=/cache/memo/release/0.9.0/src/memo/grpc/grpc/grpc_cpp_plugin src/memo/grpc/memo_vs_with_named.proto

The problem is that libprotoc has an absolute rpath pointing to
another build (because with build-end we copy build tree from one
place to another).  Once this other build destroyed, the build has no
way to resurect.

infinit/grpc,infinit/grpc,infinit/grpc,infinit/grpc,infinit/grpc,infinit/grpc,infinit/grpc,infinit/grpc,infinit/grpc