Feature request for --error-undefined-variables

Hi all:

Makefiles can become complex beasts, and option "--warn-undefined-variables" helps find programming errors.

Unfortunately, makefiles also tend to generate a lot of text, and therefore such warnings can be easily missed. This reduces the usefulness of "--warn-undefined-variables" to a great extent.

I would like to have a flag like "--error-undefined-variables" instead, which would be the equivalent to Bash' option "set -o nounset". This way, such programming errors cannot go undetected any more.

Best regards,
  rdiez