Implement basic testing infrastructure (#19)

* Add configuration for CMake build system alongside autoconf

- Add a modern CMake build system (`CMakeLists.txt`, `CMakePresets.json`) that coexists with the legacy
autoconf/Makefile build
- Shared source files (`midifile.c`, `parseabc.c`, `music_utils.c`, `parser2.c`) are compiled once via OBJECT
libraries and linked into the 8 binaries
- Three presets: `default` (Release), `debug`, `sanitize` (ASan + UBSan)
- Generates `compile_commands.json` for clangd/LSP editor support
- Install rules match the legacy Makefile (binaries, doc files, man pages)
- Pinned to `-std=gnu89` because the codebase mixes K&R `()` and ANSI typed prototypes — in C23/gnu23 (GCC 15+
default), `()` means `(void)`, making these a hard error. Note: **the existing autoconf build is also broken with
GCC 15** for the same reason

```sh
cmake --preset debug
cmake --build --preset debug
cmake --install build/debug --prefix /usr/local

Documentation

- README.md: added Building section with both autoconf and CMake instructions
- doc/readme.txt: added build instructions in the existing preamble
- doc/CHANGES: added changelog entry

Test plan

- All 3 presets configure and build with GCC 15
- Smoke test: abc2midi samples/coleraine.abc produces valid MIDI through mftext
- Sanitizer build (--preset sanitize) runs clean on sample files
- Install layout verified: 8 binaries, 10 doc files, 8 man pages in correct paths
- Build on macOS (untested, should work with AppleClang)

* Implement basic testing infrastructure

The CMake build includes a test suite covering all 8 programs:

- **Smoke tests** verify each binary runs cleanly with `-ver`.
- **Golden-file tests** run each program on a sample input and compare the
  (normalized) output to a checked-in reference. Binary MIDI outputs are
  piped through `mftext` to produce diffable text. Volatile lines (version
  banners, dates, temporary paths) are stripped before comparison.

```sh
ctest --preset debug

ctest --preset debug -L golden
ctest --preset debug -L smoke
```

To regenerate the golden files after an intentional behavioural change,
review the diff, then commit:

```sh
cmake --build build/debug --target update-golden
git diff tests/golden/
```

* Factorize more the test CMake code

doc/CHANGES

@@ -15688,3 +15688,12 @@ it has been restructured. Changes in genmidi.c (PART case in writetrack,
 partmarkers global) and store.c (event_part, event_init for -PMAR flag).
 Man page updated in doc/abc2midi.1.
 
+2026 April 01
+build: added CMake build system (CMakeLists.txt, CMakePresets.json)
+alongside the existing autoconf build.  Three presets are provided:
+default (Release), debug, and sanitize (ASan+UBSan).  Shared source
+files (midifile.c, parseabc.c, music_utils.c, parser2.c) are compiled
+once via OBJECT libraries.  The build exports compile_commands.json
+for LSP/clangd support.  Note: the codebase requires -std=gnu89 until
+the K&R-to-ANSI prototype migration is completed.
+