Run autogen.sh in release script.

Add protoc release script for Linux build.

Add cpp tests under release docker image.

1. Added C++ tests under the release docker image.
2. Fixed a compile issue with the docker image.
3. Cleaned up tests.sh and removed traivis stuff.

Update protoc build scripts.

1. Changed maven script to only do artifact uploading and removed build
script invocation from it. We didn't use maven to invoke the build
script before (we built protoc manually and editted pom.xml to only do
uploading for previous releases), and will not use it in the future (we
will use kokoro to build artifacts).
2. Cleaned up build-protoc.sh and README.md: removed the part about
using maven to build and listed supported platforms explicitly.

Fix php tests

fix linux kokoro build in docker

fix commonjs js provide GOOGLE_CHECK

Compilation of Python wrappers fails with Python 3.7 because
the Python folks changed their C API such that
PyUnicode_AsUTF8AndSize() now returns a const char* rather
than a char*. Add a patch to work around. Relates #4086.

Add scripts for automated building of C# nuget on Kokoro

Make ruby release configs consistent with protoc.

* objectivec: Quash -Wself-assign

* objectivec: Set -Wno-vla when building

Objective-C protobuf uses VLAs for performance reasons. Ensure Clang
doesn’t complain about them.

OnShutdownDelete shadowing issue

protobuf_generate(): add relative path to output dir

Without this fix, protobuf_generate() sets the variable _generated_srcs to
${protobuf_generate_PROTOC_OUT_DIR}/${_rel_dir}/${_basename}${_ext}
but generates the files in
${protobuf_generate_PROTOC_OUT_DIR}/${_basename}${_ext}

https://github.com/google/protobuf/issues/4887

* Enable ignoring unknown in json parsing in php

* Update generated descriptor files

* Update failure list for other languages.

* Remove unnecessary php files

Fix cpp benchmark dependency on mac

Merge 3.6.x into master

Fix initialization with Visual Studio

protoc-artifacts: Update centos base from 6.6 to 6.9

This avoids the need to use "yum update && yum upgrade" in the container
to be able to contact GitHub, which requires TLS 1.2[1].

I have verified that binaries built with this container still run in the
previous container; no errors like "/lib64/libc.so.6: version
`GLIBC_2.14' not found", which occur if using too new of a glibc when
compiling. CentOS 6.6 has glibc version 2.12 release 1.209.el6. CentOS
6.9 has glibc version 2.12 release 1.149.el6. Both would upgrade to
release 1.212.el6 via "yum update && yum upgrade".

1. https://githubengineering.com/crypto-deprecation-notice/

Updated Docker setup to use GCC 4.8

It appears that Visual Studio does not work well with std::once_flag
because it has a bug causing it to initialize that during dynamic
initialization instead of constant initialization. This change works
around the problem by using function static initializers instead.

@gerben-s originally wrote this change for the Google-internal codebase
but I am just cherry-picking it here.

This fixes #4773.