title: How to specify default values for environment variables in CMakePresets
body: <p>When using <code>CMakePresets.json</code> the <a href="https://cmake.org/cmake/help/latest/manual/cmake-presets.7.html#macro-expansion" rel="nofollow noreferrer">documentation</a> states that I can use either <code>$env{&lt;variable-name&gt;}</code> or <code>$penv{&lt;variable-name&gt;}</code> to query environment variables.</p>
  <p>Is it possible to specify a default value in case the environment variable is not set?</p>
  <p>Note:</p>
  <ul>
  <li>Since the variable that I am trying to set is also inside the presets, I can't handle the issue inside a CMake script.</li>
  <li>If default values are not supported is there any workaround with which I could achieve the same inside the presets file?</li>
  </ul>