configure: Let the user enable both built-in protocols and purple

There's no real risk in enabling these, since protocols/purple/purple.c
already avoids adding duplicate protocols in purple_initmodule():

    /* If we already have this one (as a native module), don't
       add a libpurple duplicate. */
    if( find_protocol( prot->info->id ) )
        continue;
Merge branch 'patch/purple-protocol-override-dx' into develop