id: adjust/restrict smack support to newer versions of libsmack

id: adjust/restrict smack support to newer versions of libsmack

There was slight change to libsmack such that positive values are
reserved for returning length of the label for smack_new_label_from_*
functions.

* m4/jm-macros.m4: Set HAVE_SMACK when both smack_new_label_from_self()
and recently added smack_new_label_from_path() are present.
The latter's presence indicates the newer API of the former.
* src/id.c (main): Check that smack_new_label_from_self() < 0,
and not just non-zero.