title: add dependency to subdirectory in CMake
body: <p>I'm building a project in CMake.
  I have to build a build utility to build a program, which has to be included with add_subdirectory().</p>
  <p>As an example,
  file structure:</p>
  <pre><code>CMakeLists.txt
  program.c
  ...
  buildutils
  └ util
    ├ util.c
    └ CMakeLists.txt
  </code></pre>
  <p>Can I write the script that builds all of the subdirectories in directory buildutils, and build program.c with a dependency of directory buildutils?</p>