即使我安装了protobuf（2.6.1）并成功测试了它的例子，当我在ubuntu 14.04上构建tensorflow（0.9）时出现以下错误。对此有何想法？
# bazel build -c opt --config=cuda //tensorflow/cc:tutorials_example_trainer
ERROR: /usr/local/tensorflow/tensorflow/cc/BUILD:61:1: error loading package
'tensorflow/core': Encountered error while reading extension file 'protobuf.bzl': no such package '@protobuf//': Error cloning repository: https://github.com/google/protobuf: cannot open git-upload-pack caused by https://github.com/google/protobuf: cannot open git-upload-pack caused by github.com and referenced by '//tensorflow/cc:tutorials_example_trainer'.
ERROR: Analysis of target '//tensorflow/cc:tutorials_example_trainer' failed; build aborted.
INFO: Elapsed time: 20.990s