title: MesonBuild: How to define dependency to a library that cannot be found by `pkg-config`?
body: <p>My project (in C) has a third party dependency at build time. But the third party library is, by default, installed to <code>/opt/</code> instead of <code>/lib</code>, and I cannot find it in <code>pkg-config</code>. From <code>mesonbuild</code>'s documentation, should I use <code>declare_dependency</code>, I don't have its source code to treat it as my sub-project. If I use <code>dependency()</code> to define it, I can't find the correct argument to define a customized location.</p>
  
  <p>How to declare dependency for a non-standard third party library?</p>