Create macro to help with deprecating options?

[kennyweiss]

Should we create a new macro to help blt users to gracefully migrate configuration options?

This macro could:

• produce a custom warning message
• indicate which option should be used instead
• optionally cause a configuration error
• ...

e.g.

blt_deprecated_option(
   NAME              foo  # option to deprecate
   MESSAGE           "Using deprecated option...."
   NEW_OPTION        bar   # use this instead
   IS_CONFIG_ERROR   false
   ...
  )