Merge master into 3.4.x

Previous code duplicates io_win32.cc in sources of all the libraries
that include io_win32.h header.

CMake: Install .pc Files (Backport: 3.4)

Add declaration for AddDescriptorsImpl()

Update version number for 3.4.0

Omit the zip test if tools are not available

Use keys() instead of iterkeys() to be python3 compatbile.

Create 3.4.x branch

AddDescriptorsImpl() is a private static implementation detail,
and needs to be declared but won't be used from outside the
source file.  Place it inside an anonymous namespace to fix a
previous build error that would result in more restrictive
build enviornments that use -Werror=missing-declarations or
similar compiler flags.

Proto2 test message support to conformance test

Windows: support long paths in open/mkdir/access

Treat missing declarations as an error for test builds, to prevent
future updates which add functions without properly declaring them.
This will prevent broken builds in more restrictive build environments.

Exclude the unconfigured .pc input files of the CMake
based install: they shall not be packed/distributed,
only their outputs are

Adds pkg-config (`.pc`) files for CMake based installs.

CMake: Install .pc Files

Exclude the unconfigured .pc input files of the CMake
based install: they shall not be packed/distributed,
only their outputs are

Adds pkg-config (`.pc`) files for CMake based installs.

Updated outdated hyperlink

Github repo had moved, pointing to correct location.

Fix typos in comment

Fixed default instance initialization for C++ lite