title: CMAKE_CURRENT_BINARY_DIR returning the wrong directory
body: <p>I have the following folder structure:</p>
  
  <pre><code>- SomeApp
    * CMakeLists.txt
    * source.cpp
    - cmake-build-debug
    - protobuf
      * CMakeLists.txt
      * file.proto
  </code></pre>
  
  <p>In my root CMakeLists.txt file is written:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.9)
  project(SomeApp)
  
  message("Root Dir: ${CMAKE_CURRENT_BINARY_DIR}")
  
  ADD_SUBDIRECTORY(protobuf)
  </code></pre>
  
  <p>And in my ./protobuf/ CMakeLists.txt is for testing just written:</p>
  
  <pre><code>message("Protobuf Dir: ${CMAKE_CURRENT_BINARY_DIR}")
  </code></pre>
  
  <p>During compiling, I get as output:</p>
  
  <pre><code>Root Dir: /home/&lt;...&gt;/SomeApp/cmake-build-debug
  Protobuf Dir: /home/&lt;...&gt;/SomeApp/cmake-build-debug
  </code></pre>
  
  <p>According to <a href="https://cmake.org/cmake/help/v3.0/variable/CMAKE_CURRENT_BINARY_DIR.html" rel="nofollow noreferrer">https://cmake.org/cmake/help/v3.0/variable/CMAKE_CURRENT_BINARY_DIR.html</a>, the actual output should be:</p>
  
  <pre><code>Root Dir: /home/&lt;...&gt;/SomeApp/
  Protobuf Dir: /home/&lt;...&gt;/SomeApp/protobuf/
  </code></pre>
  
  <p>What is going wrong with my cmake settings?</p>