Add proto2 to proto3 util

Introduce Proto C API; based on cl/198113115 by amauryfa

Add reference to protobuf-d

https://github.com/dcarp/protobuf-d is a conformance tested proto3
runtime library and code generator for D.

* Remove js_embed binary.

Upport to master: Require C++11 and pass -std=c++11

This commit updates the Autotools build to require a C++11 compiler and
explicitly pass -std=c++11. This seems to be necessary for enabling
C++11 support in older compilers.

Added Clang thread-safety annotations in mutex.h

For Clang's thread safety analysis (-Wthread-safety) to pass without warnings,
we need to annotate WrappedMutex to indicate that Lock() acquires the lock and
Unlock() releases it. This CL adds the annotations and guards them with an
ifdef to make sure they're a no-op on compilers other than Clang.

This is a cherry-pick of a change I already made to the Google-internal
codebase.

Updated generate_changelog.py

Merge 3.5.x branch into master

Added new API to GPBEnumDescriptor to enable introspection of enum values.

Refactored implementation so that this contains a minimum of added code.

Clarified comments regarding behavior in the presence of the alias_allowed option.

Added unit tests for new functionality and for the alias case.

These methods are used by grpc

Remove jruby from kokoro tests

Declare 'google' namespace when importing Python protobuf via Bazel

Add kokoro configs for bazel build.

I removed references to javanano, since that implementation no longer
exists. While I was at it I found a couple places where it looked like
the wrong directory was used by mistake and I fixed them.

In old generated code, the constructor of message doesn't provide a
default null to parent's constructor. Previously, in c extesnion, this
case was not handled.

* uses namespaces for nested messages and enums

* fixes namespaces for PHP dist

* fixes namespace for Descriptors, adds Cardinality and Kind

* fixes nested namespaces for reserved words and adds tests

* adds tests and generator fix for php class prefixes

* fixes escaping of protobuf packages, enum comments, misc others

* nice refactor of generated code

* adds class files for backwards compatibility

* simplifies code with templates

* adds compatibility files to makefile

* cleanup of generator and fixes nested namespace bug

* regenerates proto types

* remove internal BC classes

* adds deprecated warning, adds methods back

* simplifies if statement

* fixes dist files

* addresses review comments

* adds back TYPE_URL_PREFIX constant

* adds @deprecated to old nested class files

* skips tests which require a separate process when protobuf.so is enabled

* Adds tests for legacy nested classes that do not require separate processes to test

* uses legacy names for GPBUtil message check

* adds block for IDE @deprecated message

* Namespace for nested message/enum in c extension

* Remove unused code

* PHP array constructors for protobuf messages

* removes Descriptor from error message

* allows mergeFrom to accept an array

* only use initWithDescriptor if instanceof MapEntry

* adds doc comments

* removes ability for constructors to take arrays for submessages

* Revert "allows mergeFrom to accept an array"

This reverts commit b7b72182d561634af12c5c5c56a7cda3b33241f9.

* makes mergeFromArray protected and fixes mergeFrom whitespace

* Separates merging from JSON and merging from PHP array

* removes well-known types and json keys from array construction

* Addresses PR review comments

* cleans up tests

* fixes exception messages

Fix syntax error in BUILD file

`python/google/__init__.py` declares the top-level 'google' namespace so that
`google.protobuf` can be imported alongside other Google Python modules like
`google.auth`.

This works well when installing protobuf via Pip, but the Bazel `//:python_srcs`
rule doesn't include this file in its `srcs`. Bazel implicitly creates an empty
`google/__init__.py`, which does *not* set up a namespace. The result is that
consuming Python protobuf via Bazel breaks all other Google Python libraries.

This fixes #4658.

Add missing ruby/tests/test_ruby_package.proto

* CMake: Add comment for CMP0048

* CMake: osx use @rpath/ as target's install name (CMP0042)

On MacoS library should use @rpath/ as prefix path instead of absolute build path
e.g. otool -L libprotobuf.dylib
libprotobuf.dylib:
  @rpath/libprotobuf.dylib (...)
  ...

* CMake: add rpath to target for LINUX and APPLE

Fix python benchmark build

Windows: expand wildcard arguments

Fixes https://github.com/google/protobuf/issues/3957