Fix: Fetch SLJIT through the zig package manager for JIT builds

[nankishre]

Zig fetched git packages do not include submodule contents, which caused the following error while building pcre2 with JIT mode on:

So I made this tiny patch to fetch the pinned SLJIT revision through zig's package manager when JIT support is enabled and stage the fetched sources alongside pcre2_jit_compile.c. This works without modifying the C source or affecting non-JIT builds of pcre2.

[carenas]

wouldn't this require updating whenever a new version of the sljit submodule is committed?

a git clone is meant to be followed by git submodule update --init or equivalent; could zig or its package manager do that?

[nankishre]

Hi, yes the sljit commit hash would have to be updated whenever gitmodules changes aswell and I'm not aware of a helper cmd which has that feature.

Rn, the maintainers would have to run
zig fetch --save=sljit "git+https://github.com/zherczeg/sljit.git#$(git ls-tree HEAD deps/sljit | awk '{print $3}')"
every time sljit's commit # is changed. If they aren't interested in doing that maybe I could setup a workflow to check for and update the zig package that if that's okay.