message: Add comment for the dependency
diff: @@ -326,6 +326,7 @@ if(CYTHON_FOUND)
  # set sphinx-build in make files
  configure_file(doc/python3-sphinx-build.in "${CMAKE_CURRENT_BINARY_DIR}/doc/python3-sphinx-build" @ONLY)
  + # sphinx target requires gudhi.so, because conf.py reads gudhi version from it
  add_custom_target(sphinx
  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/doc
  COMMAND ${CMAKE_COMMAND} -E env "PYTHONPATH=${CMAKE_CURRENT_BINARY_DIR}"
  