title: CMake target_include_directory doesn't provide include
body: <p>I have the small project that looks like shown below:</p>
  
  <pre><code>/
   CMakeLists.txt
   src/
       CMakeLists.txt
       sources...
   libs/
        foo/
            CMakeLists.txt
            src/
                sources...
            include/
                    headers...
  </code></pre>
  
  <p>The CMakeLists.txt files look like this:</p>
  
  <p>/CMakeLists.txt:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.9)
  add_subdirectory(libs/foo)
  add_subdirectory(src)
  </code></pre>
  
  <p>/src/CMakeLists.txt:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.9)
  set(CMAKE_CXX_STANDARD 14)
  add_executable(my_app
          main.cpp
  )
  target_link_libraries(my_app ${MY_LIB})    
  </code></pre>
  
  <p>/libs/foo/CMakeLists.txt:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.9)    
  set(CMAKE_CXX_STANDARD 14)
  
  set(MY_LIB httpOnThreads)
  set(MY_LIB_INCLUDE ${CMAKE_CURRENT_SOURCE_DIR}/include)
  
  add_library(${MY_LIB} STATIC
          include/foo.h src/foo.cpp
  )
  
  target_include_directories (${MY_LIB} PUBLIC
          ${MY_LIB_INCLUDE} )
  target_include_directories(${MY_LIB} PRIVATE src)
  </code></pre>
  
  <p>I expect <code>target_link_libraries(my_app ${MY_LIB})</code> will be provide include information. But, when I try to build the project, I have fatal error: <code>No such file or directory</code></p>
  
  <p>What do I do wrong?</p>