Restored libpurple_jabber_use_builtin_digestmd5 patch, which was used in adium 1.2.x and removed in 1.3svn. It's still needed to be able to connect to certain old servers, as cyrus-sasl's digest-md5 doesn't agree with them, and I'd rather us just work than try to convince network admins to upgrade their jabber servers given that that the workaround does quite well. Fixes #10265
