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>