title: INCLUDE environment variable is empty when installing lxml
body: <p>I'm currently running python 3.4.3 and I'm trying to install lxml so I can install scrapy. When I use <code>pip install lxml</code> or try to install it from a pre-compiled .whl file the installation terminates and gives the error: INCLUDE environment variable is empty. INCLUDE is not present in the list of system environment variables, so I guess I'll have to add it manually. What should be in this variable or does someone know another way to install lxml or scrapy that would circumvent this error?
  <a href="http://i.stack.imgur.com/UQihY.png" rel="nofollow">output when trying to install lxml</a></p>