Bump gemspec again

Prefer system distributed binaries/libraries.

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

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

Protobuf now uses these two features, which are not available on some
platforms.

Update changelog for 3.4.x

Fix build on MinGW32

Make compilers without ref-qualifier support happy.

strdup is not part C or C++ standard but POXIS function. Several
compilers failed to compile strdup or generate warnings.

This excludes cygwin and mingw from using the self implemented
functions.

Make no warning test stricter.

- Now it includes -Wextra except for unused-parameter.
- Removed gtest dependency

Make code free of missing-field-initializers warnings

Merge master into 3.4.x