title: conda build fails with `>=` of jinja2 variable in `host` section of recipe
body: <p>I am using a jinja2 variable in the recipe file of a feedstock, but <code>conda build</code> fails to recognize that variable in the <code>host</code> part of the <code>requirements</code> section when used along with <code>&gt;=</code>.</p>
  
  <p>This problem does not happen in the <code>run</code> section of recipe.
  Also, in the <code>host</code> section this problem does not happen if</p>
  
  <ul>
  <li>I don't use <code>&gt;=</code> along with the variable</li>
  <li>I use the hardcoded value along with <code>&gt;=</code></li>
  </ul>
  
  <p>This is the conda recipe which fails with -</p>
  
  <p>$ cat recipe/meta.yaml</p>
  
  <pre><code>package:
    name: dummy-conda
    version: 1.1.1
  
  source:
    path: .
  
  requirements:
    host:
      - python &gt;={{python}}
    run:
      - python &gt;={{python}}
  </code></pre>
  
  <p>I have added the variable in another file -</p>
  
  <p>$ cat conda_local_config.yaml</p>
  
  <pre><code>python:
    - 3.6
  </code></pre>
  
  <p>And use this command to build</p>
  
  <pre><code>$ conda build -m conda_local_config.yaml recipe/
  </code></pre>
  
  <p>which fails with </p>
  
  <pre><code>...
  Received dictionary as spec.  Note that pip requirements are not supported in conda-build meta.yaml.
  </code></pre>
  
  <p>This is the complete failure log -</p>
  
  <pre><code>$ conda build -m conda_local_config.yaml recipe/
  No numpy version specified in conda_build_config.yaml.  Falling back to default numpy value of 1.11
  WARNING:conda_build.metadata:No numpy version specified in conda_build_config.yaml.  Falling back to default numpy value of 1.11
  Adding in variants from internal_defaults
  INFO:conda_build.variants:Adding in variants from internal_defaults
  Adding in variants from conda_local_config.yaml
  INFO:conda_build.variants:Adding in variants from conda_local_config.yaml
  Received dictionary as spec.  Note that pip requirements are not supported 
  </code></pre>
  
  <p>If I modify the recipe file to this</p>
  
  <pre><code>..
    host:
      - python {{python}}
  ..
  </code></pre>
  
  <p>or</p>
  
  <pre><code>..
    host:
      - python &gt;=3.6
  ..
  </code></pre>
  
  <p>the build works fine!</p>
  
  <p>Edit - I opened up a github issue against conda-build to get an official answer to this behavior - <a href="https://github.com/conda/conda-build/issues/3544" rel="nofollow noreferrer">https://github.com/conda/conda-build/issues/3544</a></p>