title: How do I set scons system include path
body: <p>Using scons I can easily set my include paths: </p>
  
  <pre><code>env.Append( CPPPATH=['foo'] )
  </code></pre>
  
  <p>This passes the flag</p>
  
  <pre><code>-Ifoo
  </code></pre>
  
  <p>to gcc</p>
  
  <p>However I'm trying to compile with a lot of warnings enabled.
  In particular with </p>
  
  <pre><code>env.Append( CPPFLAGS=['-Werror', '-Wall', '-Wextra'] )
  </code></pre>
  
  <p>which dies horribly on certain boost includes ... I can fix this by adding the boost includes to the system include path rather than the include path as gcc treats system includes differently.</p>
  
  <p>So what I need to get passed to gcc instead of -Ifoo is</p>
  
  <pre><code>-isystem foo
  </code></pre>
  
  <p>I guess I could do this with the CPPFLAGS variable, but was wondering if there was a better solution built into scons.</p>