third_party/protobuf: update to upstream r371

third_party/protobuf/python/README.txt

 Protocol Buffers - Google's data interchange format
 Copyright 2008 Google Inc.
 
 This directory contains the Python Protocol Buffers runtime library.
 
 Normally, this directory comes as part of the protobuf package, available
 from:
 
   http://code.google.com/p/protobuf
 
@@ skipping 45 matching lines @@
    to be a bug either in Cygwin or in Python:
      http://www.cygwin.com/ml/cygwin/2005-07/msg01378.html
    We do not know if or when it might me fixed.  We also do not know
    how likely it is that this bug will affect users in practice.
 
 5) Install:
 
      $ python setup.py install
 
    This step may require superuser privileges.
+   NOTE: To use C++ implementation, you need to export the environment variable
+   before this step. See the "C++ Implementation" section below for more
+   details.
 
 Usage
 =====
 
 The complete documentation for Protocol Buffers is available via the
 web at:
 
   http://code.google.com/apis/protocolbuffers/
+
+C++ Implementation
+==================
+
+WARNING: This is EXPERIMENTAL and only available for CPython platforms.
+
+The C++ implementation for Python messages is built as a Python extension to
+improve the overall protobuf Python performance.
+
+To use the C++ implementation, export an environment variable:
+
+  $ export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
+
+You need to export this variable before running setup.py script to build and
+install the extension.  You must also set the variable at runtime, otherwise
+the pure-Python implementation will be used. In a future release, we will
+change the default so that C++ implementation is used whenever it is available.
+It is strongly recommended to run `python setup.py test` after setting the
+variable to "cpp", so the tests will be against C++ implemented Python
+messages.
+