title: undefined Refence in CMakeLists
body: <p>I want to build the webRTC using the CMakeLists.txt and then use it in my android studio.
  I have made the CMakeLists.txt in each subdirectory.</p>
  <p><strong>SubDiretory CMakeLists:</strong></p>
  <pre><code>cmake_minimum_required(VERSION 3.4.1)
  set(SRC_FILES
          //Here are all my .cc and .cpp source files
  )
  
  
  
  add_subdirectory(audio_processing)    //this is another subdirectory in folder in which i have made a CMakeLists.txt file.
  add_library(webrtc_modules ${SRC_FILES})
  target_link_libraries(webrtc_modules webrtc_modules_audio_processing)
  
  target_include_directories(webrtc_modules PUBLIC
          //these are the directories in which I want my programs to look for the header files
  
          async_audio_processing
          audio_coding
          audio_device
          audio_mixer
          audio_processing
          congestion_controller
          desktop_capture
          include
          pacing
          remote_bitrate_estimator
          rtp_rtcp
          third_party
          utility
          video_capture
          video_coding
          video_processing
  )
  </code></pre>
  <p>I am linking this library into Main CMakeLists.txt</p>
  <p><strong>Main CMakeLists:</strong></p>
  <pre><code>cmake_minimum_required(VERSION 3.10)
  
  
  set(LOCAL_CXX_FLAGS &quot;${CMAKE_CXX_FLAGS} std=c++17&quot;)
  set(CMAKE_C_FLAGS &quot;${CMAKE_C_FLAGS} -DWEBRTC_POSIX -O0 -DWEBRTC_ANDROID&quot;)
  
  include_directories(modules)
  
  link_directories(modules)
  
  add_subdirectory(modules)
  
  
  
  
  
  set(STATIC_LIBS webrtc_modules)
  
  add_library(libwebrtc)
  
  target_link_libraries(libwebrtc PUBLIC ${STATIC_LIBS})
  </code></pre>
  <p>Library build successfully but when I call any function of this subDirectory then it gives me <strong>Undefined reference</strong>.
  Can you please tell me how to solve this issue?
  Any help in this regard will be highly appreciated.</p>