Add conf-libclang.21 using a generic configure script

[jmid]

There is a current issue with conf-libclang which has not evolved as fast as new clang versions are upgraded in distributions.

The current conf-libclang.15 is thus failing on several distributions because system packages such as libclang-15-dev are no longer available on a recent distro, causing a downgrade to conf-libclang.12 (with unversioned system package constraints) which will instead fail the version check, as "libclang.21 is not in [3;12]".

This WIP PR

• takes a first shot at a conf-libclang.21 more up to date with distribution versions and
• attempts to make the bash configure script generic, so that we don't have to submit PRs to both opam-source-archives and opam-repository to roll a new version.

AFAICS we were already passing the opam version variable to configure.sh in earlier versions, but not using it? 🤔

Here's a diff to assess what has changed in the configure script:

$ diff -db patches/conf-libclang/configure.sh.15 patches/conf-libclang/configure-generic.sh.1
9c9,20
< maximum_version=15
---
> if [ $# != 1 ]; then
>     echo "Error: Expected exactly one argument, but got $#"
>     echo "Usage: $0 <maximum-llvm-version-to-check>"
>     exit 1
> fi
> 
> if [ $1 -lt 3 ]; then
>     echo "Error: version argument should be at least 3"
>     exit 1
> fi
> 
> maximum_version=$1
130c141
< echo "Error: No usable version of LLVM <=15.0.x found."
---
> echo "Error: No usable version of LLVM <= $maximum_version.0.x found."

[jmid]

I've opened ocaml/opam-source-archives#55 with the generalized script, which should be reusable for 22, or 16, ..., 20 too, if we need them.

[jmid]

This is all green modulo

• Windows failures
• frama-clang revdeps failures

It is thus a good step forward to address conf-libclang failures.

The PR needs a maintainer to merge ocaml/opam-source-archives#55
after which I can update the URL in the opam-file.

[mseri]

Tests had already run on the file, and it was retrieved and built correctly from the new url

[mseri]

Thanks!