title: Appending CXX_FLAGS in cmake invocation
body: <p>I am wondering if it is possible to append to inferred variables when calling <code>cmake</code>. Due either the compiler or the app supplied <code>CMakeLists.txt</code> C++11 support isn't detected by <code>cmake</code> even though it is supported and the application requires it. If I do:</p>
  
  <pre><code>cmake-DCMAKE_CXX_FLAGS='-std=c++11' ../source_dir
  </code></pre>
  
  <p>The flags get overwritten and I wouldn't like to lose the inferred flags. I could manually run without overwriting and just copy the flags and append c++11 but I think there must be a better solution.</p>
  
  <p>I have found a number of posts on adding c++11 support to <code>CMakeLists.txt</code> but not in <code>cmake</code> call, so I am wondering if this is possible at all. Please let me know.</p>