title: cmake transitive private include directory
body: <p>Given this cmake sample project:</p>
  
  <p>lib1/CMakeLists.txt:</p>
  
  <pre><code>add_library(lib1 src1.cpp)
  target_include_directories(lib1 PUBLIC include)
  </code></pre>
  
  <p>lib2/CMakeLists.txt:</p>
  
  <pre><code>add_library(lib2 src2.cpp)
  target_include_directories(lib2 PUBLIC include)
  target_link_libraries(lib2 PRIVATE lib1)
  </code></pre>
  
  <p>lib3/CMakeLists.txt:</p>
  
  <pre><code>add_library(lib3 src3.cpp)
  target_include_directories(lib3 PUBLIC include)
  target_link_libraries(lib3 PRIVATE lib2)
  </code></pre>
  
  <p>All .cpp and .h files are dummy files with no dependency between them.</p>
  
  <p>From what I understood from the documentation, adding a "PRIVATE" target library from lib2 to lib1 means that lib1 include directory should not be added when compiling lib3. However, when launching compiling this using cmake (3.3.2) and "make VERBOSE=1", command line to compile "src3.cpp" contains "-I/.../lib1/include":</p>
  
  <pre><code>c++ -I/.../lib3/include -I/.../lib2/include -I/.../lib1/include -o .../src3.cpp.o -c /.../lib3/src3.cpp
  </code></pre>
  
  <p>What did I get wrong ?</p>