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>