INCLUDE environment variable is empty when lxml is set

Question

I am currently running python 3.4.3 and I am trying to install lxml to install scrapy. When I use pip install lxmlor try to install it from a precompiled .whl file, the installation completes and gives an error: INCLUDE environment variable is empty. INCLUDE is not in the list of system environment variables, so I think I will have to add it manually. What should be in this variable, or does anyone know another way to set lxml or scrapy to get around this error? output when trying to install lxml