title: What is the purpose of EXTRA_CFLAGS?
body: <p>What is the intended use of <code>EXTRA_CFLAGS</code>?</p>
  
  <p>I see it in some contexts but I've never understood why one wouldn't just append flags to <code>CFLAGS</code> instead of <code>EXTRA_CFLAGS</code>.</p>
  
  <p>I first thought there was something to do with how make has defined its implicit rules, but this did not seem to be the case. As I understand it, there are no uses of <code>EXTRA_CFLAGS</code> in the make implicit rules, correct?</p>
  
  <p>I would appreciate any enlightenment.</p>