grep: un-break building with PCRE < 8.32

grep: un-break building with PCRE < 8.32

Amend my change earlier in this series ("grep: add support for the
PCRE v1 JIT API", 2017-04-11) to un-break the build on PCRE v1
versions earlier than 8.32.

The JIT support was added in version 8.20 released on 2011-10-21, but
it wasn't until 8.32 released on 2012-11-30 that the fast code path to
use the JIT via pcre_jit_exec() was added[1] (see also [2]).

This means that versions 8.20 through 8.31 could still use the JIT,
but supporting it on those versions would add to the already verbose
macro soup around JIT support it, and I don't expect that the use-case
of compiling a brand new git against a 5 year old PCRE is particularly
common, and if someone does that they can just get the existing
pre-JIT slow codepath.

So just take the easy way out and disable the JIT on any version older
than 8.32.

The reason this change isn't part of the initial change PCRE JIT
support is to have a cleaner history showing which parts of the
implementation are only used for ancient PCRE versions. This also
makes it easier to revert this change if we ever decide to stop
supporting those old versions.

1. http://www.pcre.org/original/changelog.txt ("28. Introducing a
   native interface for JIT. Through this interface, the
   compiled[...]")
2. https://bugs.exim.org/show_bug.cgi?id=2121

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>