protobuf-cmake: CMake build files for Protocol Buffers

Written by Jesper Eskilson (jesper@eskilson.se) with contributions from
several other people.

Requirements
------------

* CMake version 2.6
* Protobuf version 2.5.0 or 2.6.1

Download links
--------------
http://www.cmake.org/cmake/resources/software.html
http://protobuf.googlecode.com/files/protobuf-2.5.0.tar.gz

Instructions
------------

The CMake variable PROTOBUF_ROOT needs to be set to the location of
the Protocol Buffers sources. When running CMake on the command line,
this can be set like this:

   $ cd build
   $ cmake /path/to/protobuf-cmake -DPROTOBUF_ROOT=/path/to/protobufs

If you specify -DDOWNLOAD_PROTOBUF=1, the script will automatically
download protobuf.

For windows users with Visual Studio, an example build script can be like this:
```bat
::put this "build-vs2013.bat" file under root folder of this repo
@echo off
set build_dir=build-vs2013
if not exist %build_dir% md %build_dir%
cd %build_dir%
cmake .. ^
    -G "Visual Studio 12 2013" ^
    -DCMAKE_INSTALL_PREFIX=%cd%/install ^
    -DPROTOBUF_ROOT=D:/lib/protobuf-2.6.1 ^
    -DCMAKE_BUILD_TYPE=Debug

cmake --build . --config Debug
cmake --build . --target install --config Debug

cmake --build . --config Release
cmake --build . --target install --config Release

cd ..
```