Provide `MAGIC_VERSION` metadata

[robo9k]

It might be helpful for consumers to know the libmagic version they link against, e.g. MAGIC_VERSION from magic.h
See robo9k/rust-magic#454 (comment)

Build scripts can be overridden, see https://doc.rust-lang.org/cargo/reference/build-scripts.html#overriding-build-scripts
This is expected and supported for magic-sys if someone wants to use neither pkg-config nor vcpkg

Thus the libmagic version would have to be communicated with build script metadata (and not generated & included code files)
When the magic-sys build script runs
https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key

cargo:version={libmagic_version}

When the script is overridden by a user

[target.x86_64-unknown-linux-gnu.magic]
metadata_version = "5xy"

Metadata might also have to be set to a fake but valid value for docs.rs, possibly approximated by using the greatest activated crate version feature "v5-xy"

pkg-config already provides Library::version
vcpkg however does not, see https://redirect.github.com/mcgoo/vcpkg-rs/pull/56

Our setup-libmagic action does something similar with external tools, i.e. also for vcpkg: pkgconf.exe --modversion libmagic

Without pkg-config {libmagic_version} could be determined using a dependency on cc

#include <magic.h>
RUST_LIBMAGIC_VERSION=MAGIC_VERSION

let mut cc = cc::Build::new();
cc.includes(include_dirs);
let expanded = cc.file("build/expando.c").try_expand()?;
// TODO: parse version, println! metadata

Then finally a dependent could use this in their own build script, e.g. in magic

let libmagic_version = env::var("DEP_MAGIC_VERSION");

This could be used to e.g. codegen a file with const MAGIC_VERSION: c_int = 547; and include! it
Note that libmagic.pc has Version as 5.47, but it might be more intuitive to match the number format from runtime magic_version

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Downsides to the above solution are; a new dependency on cc in magic-sys
and a requirement for a build script in dependents, e.g. in magic
This might still be more acceptable than runtime bindgen. Maybe regex instead of full cc?