Add brief documentation describing each warning suppression.

In protobuf-module.cmake: in addition to checking for release or debug
variants of protoc's location we should also check the configuration "noconfig".

VS2017 warns about a few additional things, for example, files that
define no symbols (which is typically the result of a platform specific
ifdef not applying on Windows).

Suppress these warnings so the build is clean on VS2017.

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

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

Add implementations of open(2), mkdir(2), stat(2),
etc. that support long paths under Windows (paths
longer than MAX_PATH in <windows.h>, which is 260
characters).

The implementations are in a separate namespace
(google::protobuf::internal::win32), so they won't
collide with the standard implementations in
<io.h>, but after importing them with `using` they
can be drop-in replacements.

Fixes https://github.com/bazelbuild/bazel/issues/2634
Fixes https://github.com/google/protobuf/issues/2891

install 'generated_message_table_driven.h' and 'metadata_lite.h', they are included when compiling the files generated using protoc.exe 3.3.0

tested only on windows with visual studio 2015 as generator

Unfortunate typo. Just one extra ',' symbol, has led to the hidden implicit behavior.

Now everything will be fine.

This allows the file path to contain spaces which is very common on
windows.

> warning C4309: static_cast': truncation of constant value

introduced in commit 9db5b11c

the deprecated atomics.