title: Adding include directories to CMake when calling it from the command line
body: <p>I'm in a situation where I should not disturb the existing CMakeLists.txt files, but I still should add some g++ system include directory to my build.</p>
  
  <p>In other words, I need <code>-isystem /path/to/my/include</code> added to my compiler flags, but when calling something like <code>cmake ..</code>.</p>
  
  <p>Maybe something like <code>cmake .. -DCMAKE_CXX_FLAGS="$CMAKE_CXX_FLAGS -isystem /path/to/my/include"</code>? Is there a way to do this?</p>