%0c support: accept and ignore %5c when -strict-formats is not used

Damien remarked that the previous commit 4.02@15275 to support %0c
(PR#6531) introduced a new static rejection of %c formats padded with
a non-zero width. They are now rejected only when -strict-formats is
activated, and otherwise ignored, as in the legacy implementation.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15359 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02