title: Append compiler flags when running make
body: <p>I would like to append flags to the compiler flags when running <code>make</code>, without altering the Makefile in anyway, e.g.</p>
  
  <pre><code>make CXX_FLAGS+='-DDEBUG'
  </code></pre>
  
  <p>The above treats "+=" as "=", so it's not the correct symbol.</p>