Synchronize camlp4-byte-only.sh with camlp4-native-only.sh to avoid camlp4 rebuild

In the commit 13296, the compilation options in camlp4-native-only
where changed to avoid a warning of the compiler. This had the
unfortunate side-effect that ocamlbuild would compile camlp4 twice
(in -byte-only and -native-only) with different options, that is
different checksums, and would therefore rebuild everything each
time. This commit mirrors the change of -native-only.sh
in -byte-only.sh (where, as far as I understand, it should have no
effect, even if it isn't needed as there is no warning to be silenced)
to avoid rebuild.

After this change, when calling "make world.opt", none of camlp4 is
rebuilt (the checksums to check that nothing changed still take
a dozen of seconds).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13983 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02