In some situations it can be helpful to separate out the location of the
log files from the location of the stamp files. For instance if you have
a continuous integration that exposes the location where log files are
stored.

This commit adds an option that allows a user to override the default
behaviour of putting the log files in STAMP_DIR called LOG_DIR. The
previous behaviour of putting the log files into the STAMP_DIR applies
if LOG_DIR is not specified.