We cannot get IMAGE_OUTPATH from the build.log, and there is currently
no other mechanism. Creating a CHECK directory was a bad idea. It should
have been created only for CHECK, but it was always created when DEBUG
was not enabled. So it's better to just issue a warning.

The purpose is being able to examine particular target interdependency
graph for a given image having been configured to avoid convoluted
dependencies (loops in particular).

The implementation is based on SHELL hook hint by John Graham-Cumming:
http://cmcrossroads.com/ask-mr-make/6535-tracing-rule-execution-in-gnu-make