exim: add package

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>

mail/exim/patches/010-allow-json-dynamic-lookup.patch

@@ -0,0 +1,11 @@
+--- a/src/drtables.c
++++ b/src/drtables.c
+@@ -662,7 +662,7 @@ addlookupmodule(NULL, &ibase_lookup_modu
+ addlookupmodule(NULL, &ldap_lookup_module_info);
+ #endif
+ 
+-#ifdef LOOKUP_JSON
++#if defined(LOOKUP_JSON) && LOOKUP_JSON!=2
+ addlookupmodule(NULL, &json_lookup_module_info);
+ #endif
+