build: downgrade clock error to a warning for buildenv

[kevans]

Summary

It's a nice error to have, but it could be the case that you need to
enter a buildenv in order to correct the condition that caused your
clock to be wrong in the first place. Let's assume that you're
sufficiently advanced to do the right thing if you know well enough to
enter an interactive buildenv and spot a warning like this.

[kevans]

Makefile

402

Obviously this isn't a great heuristic for 'interactive buildenv' since you could actually set a shell in BUILDENV_SHELL instead of make -C ..., but it seemed like a reasonable approximation.