message: Set GENERATE_TAGFILE in Doxyfile
  Allows documentation to be linked from other projects to using Doxygen with the tag:
  TAGFILES = $(SOME_PATH)/nlohmann_json.tag=https://nlohmann.github.io/json
diff: @@ -281,7 +281,7 @@ SKIP_FUNCTION_MACROS = YES
  # Configuration options related to external references
  #---------------------------------------------------------------------------
  TAGFILES =
  -GENERATE_TAGFILE =
  +GENERATE_TAGFILE = html/nlohmann_json.tag
  ALLEXTERNALS = NO
  EXTERNAL_GROUPS = YES
  EXTERNAL_PAGES = YES
  