title: How to use virtualenv in makefile
body: <p>I want to perform several operations while working on a specified virtualenv.</p>
  
  <p>For example command</p>
  
  <pre><code>make install
  </code></pre>
  
  <p>would be equivalent to</p>
  
  <pre><code>source path/to/virtualenv/bin/activate
  pip install -r requirements.txt
  </code></pre>
  
  <p>Is it possible?</p>