Steps:
Download v2.6.1 from https://github.com/google/protobuf/releases/download/v2.6.1/protobuf-2.6.1.tar.gz
export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION_VERSION=2
export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
./configure && make && make check && sudo make install
protoc --version reports 2.6.1

The first time I tried to do 'python setup.py build' it failed; I had to install setuptools which I did with:
wget https://bootstrap.pypa.io/ez_setup.py -O - | sudo python
then I continued with:

python setup.py build
python setup.py google_test
This last step fails with "cannot import name unittest_pb2", full error below.

This fails on two different Ubuntu machines (both of which had proto 2.5 / pure-Python bindings installed before), but succeeds on two MacOS X machines (Mavericks and Yosemite; again both had 2.5 already).

Any help greatly appreciated!

Full error:

running build_ext
Traceback (most recent call last):
File "setup.py", line 200, in
"Protocol Buffers are Google's data interchange format.",
File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
dist.run_commands()
File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
self.run_command(cmd)
File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
cmd_obj.run()
File "build/bdist.linux-x86_64/egg/setuptools/command/test.py", line 142, in run
File "build/bdist.linux-x86_64/egg/setuptools/command/test.py", line 122, in with_project_on_sys_path
File "/build/protobuf-2.6.1/python/google_apputils-0.4.1-py2.7.egg/google/apputils/setup_command.py", line 157, in run_tests
ok &= self._RunTestModule(file_path)
File "/build/protobuf-2.6.1/python/google_apputils-0.4.1-py2.7.egg/google/apputils/setup_command.py", line 123, in _RunTestModule
module = imp.load_module(module_name, *import_tuple)
File "google/protobuf/internal/message_test.py", line 53, in
from google.protobuf import unittest_pb2
ImportError: cannot import name unittest_pb2