Fixes https://github.com/google/protobuf/issues/3217

Export symbols used in inline functions

fixed_address_empty_string symbol is used in an inline function.
We have to export it to avoid undefined reference link errors.

Ensure that for Java, imports of .proto files with empty packages works

Add continuous testing config files for Kokoro

Add MacOS and Linux tests to Kokoro

ObjC: Preserve unknown fields in proto3 syntax files.

Refactor cc options in BUILD file for Windows

CMake: Add modern protobuf_generate

As announced: https://groups.google.com/forum/#!topic/protobuf/VX5qEmTW3y0

The ObjC side of https://github.com/google/protobuf/issues/272

Properly error on a tag with field number zero.

Fix CMake version check

* Add new file option php_namespace.

Use this option to change the namespace of php generated classes.
Default is empty. When this option is empty, the package name will be
used for determining the namespace.

* Uncomment commented tests

* Revert gdb test change

* Update csharp descriptor.

* Add test for empty php_namespace.

This fixes a compiler bug that caused a Java syntax error when one .proto file
would import another one with an empty package and java_package. This fixes
issue #3114.

Optimize GPBDictionary.m codegen to reduce size of library

Fix some cases of reading of 64bit map values.

Fixes https://github.com/google/protobuf/issues/3164.

Don't put gcc warnings options in copts, so that protobuf is able to
build by MSVC toolchain without python wrappers.

* not to use std::random_device for map.Seed().

* remove include random

Add headers as part of cmake project generation

tested only on windows with visual studio 2015 as generator

Improve fix for https://github.com/google/protobuf/issues/295

This has one important packaging change: the netstandard version now
depends (implicitly) on netstandard1.6.1 rather than on individual
packages. This is the preferred style of dependency, and shouldn't
affect any users - see http://stackoverflow.com/questions/42946951
for details.

The tests are still NUnit, but NUnit doesn't support "dotnet test"
yet; the test project is now an executable using NUnitLite. (When
NUnit supports dotnet test, we can adapt to it.)

Note that the project will now only work in Visual Studio 2017 (and
Visual Studio Code, and from the command line with the .NET Core
1.0.0 SDK); Visual Studio 2015 does *not* support this project file
format.

* Comment out unused arguments.

These last few are all that's needed to compile with -Wunused-arguments.

* Fix missing struct field initializer.

With this fix, everything compiles with -Wmissing-field-initializers.

* Add support for disabling unaligned memory accesses on x86 too.

ubsan doesn't like these because they are technically undefined
behavior, so -DGOOGLE_PROTOBUF_DONT_USE_UNALIGNED will disable them easily.

* Avoid undefined integer overflow.

ubsan catches all of these.

Fix Markdown formatting in README.

Show help if protoc is called without any arguments