At least don't disable Twitter anymore for libpurple builds. libpurple
doesn't have Twitter support by default and it's unlikely that someone will
prefer libpurple-twitter over BitlBee's Twitter code anyway.
Also automatically disable libpurple protocol modules if a native variant is
already set up. The linking conflict works the other way around (libpurple
modules may break if native modules for the same protocol are compiled in)
so this should mostly resolve the linking conflict problem.
