title: Makefiles: What is an order-only prerequisite?
body: <p>The GNU make manual introduces <a href="https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html" rel="noreferrer">order-only prerequisites</a>:</p>
  <pre><code>target: prerequisite1 prerequisite2 | orderonlyprerequisite1 orderonlyprerequisite2
  </code></pre>
  <p>However, while that manual page gives an example and describes the purpose of order-only prerequisites, at no point does the manual page answer the question in the title: <strong>What is an order-only prerequisite?</strong></p>
  <p>Can you give a precise definition how order-only prerequisites behave?</p>