The __frontend variable was introduced to address the needs
of alterator-mkimage module: list the images available in
one column, purge the builddir.

Looks like we should consider other cases with redirected
stdout (cron builds, piped calls, etc) like fundamentally
non-interactive and behave the same.

So commit 3a8af6b5's
description is wrong now; the current cleanup rules are:

- if CLEAN=0 or DEBUG>1, don't do it;
- if CHECK or REPORT is set, don't do it;
- otherwise if at least one of the following conditions is true:
  + there's more than one target being built in a row;
  + stdout was redirected (cronjob, alterator-mkimage...);
  + metaprofile directory is read-only
  ...then do a distclean.

If that doesn't suit your needs, describe the particular
situation please.

Thanks cas@ for wondering aloud whether greppable output
is unsupported with `make help'.