title: Why am I warned that the "CMAKE_TOOLCHAIN_FILE" variable is not used by the project?
body: <p>When I build some third-party code, I am seeing the following warning from CMake:</p>
  <pre><code>CMake Warning:
    Manually-specified variables were not used by the project:
  
      CMAKE_TOOLCHAIN_FILE
  </code></pre>
  <p>What is causing this warning? I checked the configuration files, but could not find anywhere where this variable is defined.</p>
  <p>Should I be concerned about this warning? How can I fix it so that the warning goes away?</p>