mail-mta/exim-4.93.0.4: reflect DANE default in USE-flags

mail-mta/exim-4.93.0.4: reflect DANE default in USE-flags

As pointed out by Hans F. Nordhaug in bug #712360, the configuration for
Exim has changed to default to DANE, because TLS (ssl) is enabled by
default as well.  We handled the latter already, but didn't do the
former, resulting in build failures with TLS disabled.

Rework the logic such that we USE-enable dane (to reflect upstream) and
that we disable it when not enabled.

Closes: https://bugs.gentoo.org/712360
Package-Manager: Portage-2.3.89, Repoman-2.3.20
Signed-off-by: Fabian Groffen <grobian@gentoo.org>