title: Cmake include_directories()
body: <p>This is my project tree:</p>
  
  <pre><code>project
  | + src
  | + external
  | | + foo
  | | | + include
  | | | | - foo.hpp
  | | | + src
  | | | | - foo.cpp
  | | | | - CMakeLists.txt
  | | | - CMakeLists.txt 
  | | + CMakeLists.txt 
  | + src
  | | - main.cpp
  | - CMakeLists.txt
  </code></pre>
  
  <p>foo.cpp includes foo.hpp:</p>
  
  <pre><code>// foo.cpp
  #include "foo.hpp"
  </code></pre>
  
  <p>Now the problem is that including the directory in the top CMake successfully find foo.hpp, 
  but if I include in the subproject it doesn't. Any reason for it? (directories are included before the executable is compiled).</p>
  
  <pre><code>// project/CMakeLists.txt
  include_directories(external/foo/include) //OK
  add_subdirectory(external)
  
  add_executable(main main.cpp)
  target_link_libraries(main foo)
  
  // project/external/CMakeLists.txt
  add_subdirectory(foo)
  
  // project/external/foo/CMakeLists.txt 
  include_directories(include) // NOT WORKING
  add_subdirectory(src)
  
  // project/external/foo/src/CMakeLists.txt 
  add_library(foo foo.cpp)
  </code></pre>