title: Conditional variable assignment operator portability?
body: <p>According to make's <a href="http://www.gnu.org/software/make/manual/make.html#Setting" rel="nofollow">Setting Variables</a>, I can conditionally assign to a undefined variable with:</p>
  
  <pre><code>FOO ?= bar
  </code></pre>
  
  <p>Is the operator portable? Can it be used on non-GNU systems?</p>