Add a CMake build alongside autotools

[matejk]

Summary

Adds a CMake build (>= 3.18) that coexists with the autotools build, plus CI updates. Rebased onto the v2.4.8.4 release on main; 4 commits:

• VERSION file: the package version moves from AC_INIT to a top-level VERSION file read by both build systems, so there is a single source of truth. It tracks the release version (2.4.8.4), and the autotools PACKAGE_VERSION string is unchanged.
• Reusable message-compile script: messages/make_message_src.sh accepts optional source/output directories and honors $GENRB/$PKGDATA, so an out-of-source build can call it; the one-argument form keeps the historical behavior and messages/Makefile.am is unchanged.
• CI: a new workflow configures, builds, and runs ctest. Version updates for the existing workflows are in ci: update deprecated GitHub Actions versions #622.
• The CMake build itself.
• C11: a dedicated commit raises the CMake build to the C11 standard (gnu11).

Scope

Linux is fully wired: libltfs, the ltfs/mkltfs/ltfsck executables, the sg/file/itdtimg tape backends, both I/O schedulers, both key managers. macOS additionally builds the iokit tape backend. The FreeBSD/NetBSD backends are skipped with a notice and remain autotools-only. The build targets libfuse 2, matching autotools; the FUSE 3 selection follows as a small extension once #603 merges.

The CMake build compiles as C11 (gnu11, since LTFS uses _GNU_SOURCE/asprintf); a commented-out C17 line is provided for when the minimum supported toolchain is GCC 8+/Clang 7+.

Notable differences from the autotools implementation:

• ICU is found through CMake's ICU module instead of the icu-config tool that ICU removed in 2018 (ICU6x is set automatically for ICU >= 60); libxml2 uses find_package(LibXml2); fuse/uuid/net-snmp use pkg-config imported targets.
• The per-backend source symlinks and the CRC_OPTIMIZE rule are replaced by direct source paths and one SSE4.2-flagged OBJECT library.
• Plugins are MODULE libraries named exactly as ltfs.conf expects (libtape-sg.so etc.), installed to <libdir>/ltfs.
• ctest registers tests/t/*.sh by glob via the shared harness, so the build works with or without the integration test suite (Add an integration test suite using the file tape backend #611) and picks up its tests automatically.

Verification

Ubuntu VM: the CMake build and all 14 integration tests pass via ctest (with #611 merged alongside); the install tree matches autotools (bin, lib, plugins, etc/ltfs.conf{,.local}, pkgconfig, man, headers); ltfs --version reports the identical string; the autotools build and make check stay green. macOS 26 (arm64, macFUSE 5.2 SDK): the CMake build produces libltfs, the executables, and all plugins including libtape-iokit.so, as C11.

Fixes #521.

[vandelvan]

Hi @matejk ! Thanks for contributing, Just a small comment

[matejk]

Done, cmake.yml now uses actions/checkout@v6. The version updates for the existing workflows moved to a separate PR, #622.

[vandelvan]

LGTM! Thank you