title: CMake header files not being found in include
body: <p>I have library, AprilTags, that uses cmake top build it.</p>
  
  <p>I have another project AIV, that uses AprilTags. I want to keep the apriltags library inside of ~/aiv/apriltags but have another file, front_back_camera_demo that uses some of the files inside of AprilTags library.</p>
  
  <p>So the file structure looks like</p>
  
  <pre><code>~/aiv/build/
       /apriltags/CMakeLists.txt
       /apriltags/AprilTags/TagDetector.h
       /apriltags/AprilTags/*.h
       /front_back_camera_demo.cpp
       /CMakeLists.txt
  </code></pre>
  
  <p>When I run cmake on the top level CMakeLists.txt, it builds the AprilTags library successfully, but then I get a</p>
  
  <blockquote>
    <p>front_back_camera_demo.cpp:72:35: fatal error: AprilTags/TagDetector.h: No such file or directory</p>
  </blockquote>
  
  <p>error on the line where I include AprilTags/TagDetector.h</p>
  
  <p>Here are the two relevant CMakeLists.txt:</p>
  
  <p>Top level:</p>
  
  <pre><code>cmake_minimum_required(VERSION 2.6)
  
  project(AIV)
  
  add_subdirectory(apriltags)
  add_executable(front_back_camera_demo front_back_camera_demo.cpp 
  Serial.cpp)
  target_link_libraries(front_back_camera_demo apriltags)
  </code></pre>
  
  <p>Inside apriltags:</p>
  
  <pre><code>cmake_minimum_required(VERSION 2.6)
  project(apriltags)
  
  #add_definitions(-pg) #"-fopenmp)
  
  # pull in the pods macros. See cmake/pods.cmake for documentation
  set(POD_NAME apriltags)
  include(cmake/pods.cmake)
  
  file(GLOB SOURCE_FILES "src/*.cc")
  include_directories(AprilTags . /opt/local/include)
  add_library(apriltags ${SOURCE_FILES})
  
  find_package(OpenCV)
  include_directories(${OpenCV_INCLUDE_DIRS})
  target_link_libraries(apriltags ${OpenCV_LIBS}) #-pg) #-fopenmp)
  pods_use_pkg_config_packages(apriltags eigen3)
  
  if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
    target_link_libraries(apriltags -L/opt/local/lib/) # MacPorts 
  special treatment...
  else (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
    pods_use_pkg_config_packages(apriltags libv4l2)
  endif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
  
  pods_install_libraries(apriltags)
  
  file(GLOB header_files "AprilTags/*.h")
  pods_install_headers(${header_files} DESTINATION AprilTags/)
  
  pods_install_pkg_config_file(apriltags
      LIBS -lapriltags
      REQUIRES eigen3 opencv2
      VERSION 1.0.0)
  
  add_subdirectory(example)
  </code></pre>
  
  <p>What am I doing wrong?</p>