Add more capabilities to the CMake build (#4163)

This commit includes changes that enable the following in the CMake build:
* Building protobuf from source.
* Building grpc from source.
* Building the TensorFlow distributed runtime.
* Building the TensorFlow Python bindings (excluding tf.contrib).

This commit also includes minor changes to remove the implicit
dependency between a CPU-only build of the runtime and stream executor,
and makes minor changes for compatibility with Protobuf 3.0.0 (regarding
compatibility between Protobuf's and TensorFlow's int64 representation).