title: Add only headers of an imported module to a library in CMake
body: <p>In CMake there are imported modules that are used to simply add external modules to local targets. For example if we want to use <code>boost::filesystem</code> library in our project we could have a <code>CMakeLists.txt</code> like this:</p>
  
  <pre><code>project(foo CXX)
  
  find_packge(Boost REQUIRED COMPONENTS filesystem)
  
  add_executable(foo main.cpp)
  target_link_libraries(foo Boost::filesystem)
  </code></pre>
  
  <p>With above configuration CMake will add proper compiler options and include directories among required libraries to building process of the <code>foo</code>.</p>
  
  <p>Now we have to build a library instead of an executable and we don't want to link <code>boost::filesystem</code> libraries to our library. We want only compiler options and include directories to be added to our target. Could we use imported modules concepts here? I mean that if we could use <code>Boost::filesystem</code> syntax for adding those options to our target?</p>
  
  <pre><code>project(foo CXX)
  
  find_packge(Boost REQUIRED COMPONENTS filesystem)
  
  add_library(foo STATIC foo.cpp)
  # what should be wrote here to only add headers and configs to foo not the libs?
  </code></pre>