title: Use environment variable in PyDev Eclipse project file
body: <p>I am using Eclipse 3.6.1 with PyDev 1.6.4 for developing a Google App Engine site.  In my projects <code>.pydevproject</code> I want to set the path to the Google App Engine SDK based off an environment variable since I develop on my desktop and laptop (where the paths are slightly different) and I plan to check this file into source control and others environment may be different. In the XML below, I want to replace <code>/home/jesse</code> with the value read from an environment variable. For now I would be happy just using the current home directory.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8" standalone="no"?&gt;
  &lt;?eclipse-pydev version="1.0"?&gt;
  &lt;pydev_project&gt;
    &lt;pydev_property name="org.python.pydev.PYTHON_PROJECT_INTERPRETER"&gt;Python 2.5&lt;/pydev_property&gt;
    &lt;pydev_property name="org.python.pydev.PYTHON_PROJECT_VERSION"&gt;python 2.5&lt;/pydev_property&gt;
    &lt;pydev_variables_property name="org.python.pydev.PROJECT_VARIABLE_SUBSTITUTION"&gt;
      &lt;key&gt;GOOGLE_APP_ENGINE&lt;/key&gt;
      &lt;value&gt;/home/jesse/projects/google_appengine&lt;/value&gt;
    &lt;/pydev_variables_property&gt;
    &lt;pydev_pathproperty name="org.python.pydev.PROJECT_SOURCE_PATH"&gt;
      &lt;path&gt;/app_html5rest&lt;/path&gt;
    &lt;/pydev_pathproperty&gt;
    &lt;pydev_pathproperty name="org.python.pydev.PROJECT_EXTERNAL_SOURCE_PATH"&gt;
      &lt;path&gt;${GOOGLE_APP_ENGINE}&lt;/path&gt;
      &lt;path&gt;${GOOGLE_APP_ENGINE}/lib/django&lt;/path&gt;
      &lt;path&gt;${GOOGLE_APP_ENGINE}/lib/webob&lt;/path&gt;
      &lt;path&gt;${GOOGLE_APP_ENGINE}/lib/yaml/lib&lt;/path&gt;
    &lt;/pydev_pathproperty&gt;
  &lt;/pydev_project&gt;
  </code></pre>
  
  <p>I have tried:</p>
  
  <ul>
  <li>${HOME}</li>
  <li>${env.HOME}</li>
  <li>$HOME</li>
  </ul>
  
  <p>Google has not turned up much, any suggestions?</p>