title: scons adding --prefix option
body: <p>What is the typical use-case for adding <code>--prefix</code> support to a scons build?</p>
  
  <p>Currently I use Variables to persist user options from the command-line, but that requires I use the name <code>PREFIX</code> to be recognized as a variable. The docs show a way to specify the preifx using <code>AddOption</code> but then this option must be specified everytime scons is run: it isn't persisted like the variables.</p>
  
  <p>What is the typical way to handle this?</p>