net-im/jabber: unbreak with libc++ 3.9

log.cc:89:8: error: assigning to 'char *' from incompatible type 'const char *'

pos = strchr(zone,'.');
      ^~~~~~~~~~~~~~~~

xmlnode.cc:913:21: error: assigning to 'char *' from incompatible type 'const char *'

start_predicate = strchr(path, '[');
                ^ ~~~~~~~~~~~~~~~~~

xmlnode.cc:914:15: error: assigning to 'char *' from incompatible type 'const char *'

next_step = strchr(path, '/');
          ^ ~~~~~~~~~~~~~~~~~

xmlnode.cc:1836:27: error: read-only variable is not assignable

strchr(lang, '-')[0] = 0;
~~~~~~~~~~~~~~~~~~~~ ^

Reported by: pkg-fallout