title: CMake for Doxygen
body: <p>I am trying to use CMake for Doxygen generated documentation. This is what my CMakeList.txt looks like:</p>
  
  <pre><code>if (DOXYGEN_FOUND)
      # set input and output files
      set(DOXYGEN_IN ${CMAKE_CURRENT_SOURCE_DIR}/config-file)
      set(DOXYGEN_OUT ${CMAKE_CURRENT_BINARY_DIR}doc)
  
      # request to configure the file
      configure_file(${DOXYGEN_IN} ${DOXYGEN_OUT} @ONLY)
      message("Doxygen build started")
  
      # note the option ALL which allows to build the docs together with the application
      add_custom_target( doc_doxygen ALL
          COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}
          WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
          COMMENT "Generating API documentation with Doxygen"
          VERBATIM )
  else (DOXYGEN_FOUND)
    message("Doxygen need to be installed to generate the doxygen documentation")
  endif (DOXYGEN_FOUND)
  </code></pre>
  
  <p>Upon running <code>make</code> , I get these errors:</p>
  
  <pre><code>Doxygen build started
  -- Configuring done
  -- Generating done
  -- Build files have been written to: /home/newproject/build
  [  5%] Generating API documentation with Doxygen
  warning: tag INPUT: input source `doc/mainpage.txt' does not exist
  warning: tag INPUT: input source `src/player.cpp' does not exist
  warning: tag INPUT: input source `src/player.h' does not exist
  warning: tag INPUT: input source `test/tests-mainfunctionality-v2.cpp' does not exist
  error: tag OUTPUT_DIRECTORY: Output directory `doc' does not exist and cannot be created
  Exiting...
  CMakeFiles/doc_doxygen.dir/build.make:57: recipe for target 'CMakeFiles/doc_doxygen' failed
  make[2]: *** [CMakeFiles/doc_doxygen] Error 1
  CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/doc_doxygen.dir/all' failed
  make[1]: *** [CMakeFiles/doc_doxygen.dir/all] Error 2
  Makefile:83: recipe for target 'all' failed
  make: *** [all] Error 2
  </code></pre>
  
  <p>even though these files do exist. Is there something wrong with the paths I am using?</p>