title: How to use add_dependency with CMake to non-subdirectories?
body: <p>I have the following structure:</p>
  
  <pre><code>${root}
      project_a
          main.c
          CMakeLists.txt
      project_b
          lib.c
          target.cmake
  </code></pre>
  
  <p>CMakeList.txt</p>
  
  <pre><code>include(${ROOT}/project_b/target.cmake)
  
  add_executable(${PROJECT_NAME} main.c)
  add_dependencies (${PROJECT_NAME} app_cmake_b)
  target_link_libraries (${PROJECT_NAME} app_cmake_b)
  </code></pre>
  
  <p>target.cmake</p>
  
  <pre><code>add_library(project_b STATIC ${ROOT}/lib.c)
  </code></pre>
  
  <p>Is using include the only way to import other cmake projects? </p>
  
  <p>I do not like that I need absolute paths in my included <code>target.cmake</code>
  What is the best pratice to solve my usecase with CMake?</p>