title: pip requirement.txt conditionals or environment markers based on env variables
body: <p>Is there a way to specify conditional installs in a pip requirements.txt file that are based on the value of an environment variable?</p>
  
  <p>I've been able to control most of what I need with environment markers, but all the markers I know of work only with specific values that are essentially pre-defined by python.</p>
  
  <p>For example, I want to be able to control package installation for RHEL 5.3 vs. RHEL 6.3 vs. RHEL 6.6, etc.  Also based on other criteria.  </p>
  
  <p>It would be perfect if I could specify in the results.txt file an environment variable that would be checked against a value I set before running pip. This seems like it would be desirable and straight forward functionality.  I haven't had much luck so far finding comprehensive discussions of environment markers, so I'm hoping I've just missed some key bit of information :-)</p>
  
  <p>Thanks much.</p>