* 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

CMakeLists.txt adds c++11 flags for clang

cf https://cmake.org/cmake/help/latest/policy/CMP0048.html#policy:CMP0048

Add brief documentation describing each warning suppression.

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.

> warning C4309: static_cast': truncation of constant value

introduced in commit 9db5b11c

A series of improvements:

- Improved Protobuf module compatibility (disabled by default);
- Hide advanced settings;
- Added build tree configuration;
- Added build of examples.

Additional export as "protobuf" package
for importing from other cmake projects

Now the Build tool needs to define -DHAVE_ZLIB and -DHAVE-PTHREAD rather
than providing a config.h

- Make pbconfig.h a manually written file to handle hash conditions
  according to platform related macros.
- Remove #include "config.h" from source code.
- Changed the configure.ac and Makefile.am to pass down the macros.
- Change cmake to pass down the the macros.

Change-Id: I537249d5df8fdeba189706aec436d1ab1104a4dc

Make it executable and generate extract_includes.bat in the same directory.