Simplify CMAKE

Remove stuff from CMakeFiles that is not used.

I wonder which parts here are useful and working. This is the last part of the root CMakeLists.txt.

Please mark those that can be removed:

• Export

  export(PACKAGE ${PLUGIN_NAME})

• DOxygen

  FIND_PACKAGE(Doxygen)
  
  FIND_PATH(IRI_DOC_DIR doxygen.conf ${CMAKE_SOURCE_DIR}/doc/iri_doc/)
  IF (IRI_DOC_DIR)
    ADD_CUSTOM_TARGET (doc ${DOXYGEN_EXECUTABLE} ${CMAKE_SOURCE_DIR}/doc/iri_doc/doxygen.conf)
  ELSE (IRI_DOC_DIR)
    ADD_CUSTOM_TARGET (doc ${DOXYGEN_EXECUTABLE} ${CMAKE_SOURCE_DIR}/doc/doxygen.conf)
  ENDIF (IRI_DOC_DIR)

• Distclean

  ADD_CUSTOM_TARGET (distclean @echo cleaning cmake files)
  
  IF (UNIX)
    ADD_CUSTOM_COMMAND(
      COMMENT "distribution clean"
      COMMAND make ARGS clean
      COMMAND rm ARGS -rf ${CMAKE_SOURCE_DIR}/build/*
  
      TARGET  distclean
      )
  ELSE(UNIX)
    ADD_CUSTOM_COMMAND(
      COMMENT "distclean only implemented in unix"
      TARGET  distclean
      )
  ENDIF(UNIX)

• Uninstall

  ADD_CUSTOM_TARGET (uninstall @echo uninstall package)
  
  IF (UNIX)
    ADD_CUSTOM_COMMAND(
      COMMENT "uninstall package"
      COMMAND xargs ARGS rm < install_manifest.txt
      TARGET  uninstall
    )
  ELSE(UNIX)
    ADD_CUSTOM_COMMAND(
      COMMENT "uninstall only implemented in unix"
      TARGET  uninstall
    )
  ENDIF(UNIX)