Fixing unused parameter warnings under Clang.

Merge 3.4.x into master

* remove the parens from the cmp() lambda definition

https://github.com/PythonCharmers/python-future/pull/298

* remove the parens from the cmp() lambda definition

https://github.com/PythonCharmers/python-future/pull/298

Bump gemspec again

Prefer system distributed binaries/libraries.

protoc-artifacts: Bump JDK to 8u131

This reverts commit 703f414c.

* Drop Python 3.3 from testing & add Python 3.5, 3.6

Python 3.3 goes [EOL](https://docs.python.org/devguide/index.html#branchstatus) next month.

* Sync test.sh with the Python versions in tox.ini

* Define cmp() for Python 3

http://python-future.org/compatible_idioms.html?highlight=cmp#cmp

* Define cmp() for Python 3

http://python-future.org/compatible_idioms.html?highlight=cmp#cmp

Python 3 compatibility fixes: print(), long(), etc.

The future import is required to support the print(x, end=" ") parameter in Python 2.

http://python-future.org/compatible_idioms.html#file

Old style exception --> new style exception

Bump gem version for the next upload

Update testpypi URLs

It seems like the image has a /usr/local/bin/autoconf installed, which
doesn't recognize/work with the yum installed libtools. Putting
distributed binaries/libraries first solves the problem

The old one is deprecated and no longer working.

Add std::forward and std::move autoconf check

Fixing -Wexpansion-to-defined Clang warning

Static link pthread for mingw64 protoc and enable cross compile

Fix cmath/math.h include with non C++11 libstdc++

Need to install mingw-w64 on linux to cross compile