Add Exim MTA variants:
* exim
plain variant without any TLS library which hence comes
without TLS, DANE and DKIM.
* exim-openssl
linked against libopenssl
* exim-gnutls
linked against libgnutls
* exim-ldap
linked against libopenssl, libopenldap and libsasl2
Provide packages for lookup modules
* cdb
* dbmdb
* dnsdb
* json (depends on jansson)
* mysql (depends on libmariadb)
* passwd
* pgsql (depends on libpq)
* redis (depends on libhiredis)
* sqlite (depends on libsqlite3)
Note:
As gnutls requires libunbound which depends on libopenssl to provide
libgnutls-dane, disable DANE by default when building with gnutls.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
