title: Why can't I get `pip install lxml` to work within a virtualenv?
body: <p>Note: I'm using virtualenvwrapper.</p>
  
  <p>Before activating the virtual environment:</p>
  
  <pre><code>$ pip install lxml
  Requirement already satisfied (use --upgrade to upgrade): lxml in /usr/lib/python2.7/dist-packages  
  Cleaning up...  
  </code></pre>
  
  <p>After activating the virtual environment:</p>
  
  <pre><code>(test-env)$ pip install lxml
  force/build/lxml/src/lxml/includes/etree_defs.h:9:31: fatal error: 
  libxml/xmlversion.h: No such file or directory
  
  compilation terminated.
  
  error: command 'gcc' failed with exit status 1
  
  ----------------------------------------
  Command /home/chaz/dev/envs/test-with-system-python-force/bin/python2
  .7 -c "import setuptools;__file__='/home/chaz/dev/envs/test-with-
  system-python-force/build/lxml/setup.py';exec(compile(open(__file__).
  read().replace('\r\n', '\n'), __file__, 'exec'))" install --record 
  /tmp/pip-bJ6Q_B-record/install-record.txt --single-version-externally
  -managed --install-headers /home/chaz/dev/envs/test-env/include/site/python2.7 failed with error code 1 in 
  /home/chaz/dev/envs/test-env/build/lxml
  Storing complete log in /home/chaz/.pip/pip.log
  </code></pre>