Use pkg-config

[robo9k]

This implements #1 and should solve the long standing issues of linking on macOS and statically in general.

This is a (potentially) breaking change for several reasons:

• MAGIC_DIR, <TARGET>_MAGIC_DIR, MAGIC_STATIC, <TARGET>_MAGIC_STATIC environment variables are no longer supported
  users need to configure pkg-config or vcpkg instead
• libmagic needs to be version 5.39 or newer
  this is required to have a libmagic.pc for pkg-config, see Use pkg-config #1 (comment)
• the old version feature flags v5-38 and below no longer make sense and need to be removed, see New requirements for libmagic, pkg-config, brew & vcpkg #36 (comment)

Related issues:

• Investigation of pkg-config, brew / vcpkg on GHA #35 looks at how GitHub Actions could work with those new requirements
  This pull request here does not update the workflows yet
• New requirements for libmagic, pkg-config, brew & vcpkg #36 looks at the results of the changed crate requirements
  Ubuntu 20.04.6 LTS Focal Fossa is no longer supported with its default packages

Misc:

• the new pkg-config crate dependency has a MSRV of v1.30.0, so we don't need to bump ours

Follow-up tasks: (not in this pull request)

• Remove old libmagic version crate features
  see Remove obsolete libmagic version features #47
• Make both pkg-config and vcpkg into default-features,
  see Make pkg-config and vcpkg default crate features #40
• CI workflows for supported configurations (e.g. Windows / Linux / macOS, static / dynamic, pkg-config / vcpkg),
  see Add and use setup-libmagic GitHub Action #41
  see Add Cargo.lock #44
  see Run CI build workflow on default branch for caching #45
  see Build extra CI scenarios #46
  see Add CI workflow to test upstream libmagic versions #48
• Document CI workflows so others can re-use them
• Release & publish new crate version
• Use new crate version in magic crate

[robo9k]

The MACIG_LINK=auto|dynamic|static env config in #35 (comment) is not actually easily possible (with vcpkg), thus it is not implemented.