Generic rules like:

  %: %.in

have the downside that the source (%.in) itself matches the target
(via %.in: %.in.in). This leads to things like:

  Looking for a rule with intermediate file `doc/examples/hooks/ipsec.in.in'.
   Trying pattern rule with stem `ipsec.in.in'.
   Trying implicit prerequisite `doc/examples/hooks/ipsec.in.in.in'.
   Trying pattern rule with stem `ipsec.in.in'.
   Trying implicit prerequisite `doc/examples/hooks/ipsec.in.in.in'.
   Looking for a rule with intermediate file `doc/examples/hooks/ipsec.in.in.in'.
  Rejecting impossible implicit prerequisite `doc/examples/hooks/ipsec.in.in'.

To fix this, we need to tell make that such rules are terminating, so
that it doesn't recurse into them.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>