title: Conditionally enable Jenkins declarative pipeline options?
body: <p>Does Jenkins provide any functionality to achieve the following pipeline described below?</p>
  
  <pre><code>pipeline
  {
      agent any
      options
      {
          when {
              branch 'master'
          }
          disableConcurrentBuilds()
      }
      stages { ... }
  }
  </code></pre>
  
  <p>I have two states for repositories which this single pipeline must manage:</p>
  
  <ol>
  <li>Build for commits to merge-requests branches (<em>pre-merge</em>), allow builds to be run concurrently</li>
  <li>Build the master branch on merge of merge-requests (<em>post-merge</em>), do not allow builds to be run concurrently.</li>
  </ol>