message: Avoid clash of duplicate uninstall target
  This can happen if CAF is integrated into a larget cmake build
  through add_subdirectory(). In case another project in the
  superbuild defines a target with the same name, cmake aborts.
diff: @@ -404,12 +404,15 @@ install(DIRECTORY libcaf_test/caf/ DESTINATION include/caf
  configure_file("${CMAKE_CURRENT_SOURCE_DIR}/cmake/cmake_uninstall.cmake.in"
  "${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake"
  IMMEDIATE @ONLY)
  -# add uninstall target
  -add_custom_target(uninstall
  +# add uninstall target if it does not exist yet
  +if(NOT TARGET uninstall)
  + add_custom_target(uninstall)
  +endif()
  +add_custom_command(TARGET uninstall
  + PRE_BUILD
  COMMAND "${CMAKE_COMMAND}" -P
  "${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake")
  -
  ################################################################################
  # set inclue paths for subprojects #
  ################################################################################
  