title: CMake add_definitions does not seem to work
body: <p>I'm trying to define a preprocessor variable via CMake, but unfortunately it does not seem to work.</p>
  
  <p>The following is my c++ code:</p>
  
  <pre><code>     #ifdef hehe_test
       #define it_exists "it_exists"
       #endif
  
       #ifndef hehe_test
       #define it_exists "it_doesnt_exist"
       #endif
  
       int main(int argc, char** argv)
       {
          printf("%s\n",it_exists);
  
          return 0;
      }
  </code></pre>
  
  <p>And this is my CMakeLists.txt:</p>
  
  <pre><code>    project(LibraryTester)
  
      add_definitions(hehe_test)
  
      file(GLOB src "*.h" "*.cpp")
  
      include_directories(${CMAKE_SOURCE_DIR}/include)
  
      add_executable( ${PROJECT_NAME} ${src})
  </code></pre>
  
  <p>When i run my program it outputs: it_doesnt_exist <br/>
  indicating "hehe_test" has not been defined.</p>
  
  <p>What am i doing wrong here?</p>