Add configuration for CMake build system alongside autoconf (#17)

- 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)

Co-authored-by: Seymour Shlien <fy733@ncf.ca>

README.md

@@ -59,30 +59,3 @@ Available presets:
 The CMake build generates `compile_commands.json` for use with
 clangd and other LSP-based editors.
 
-### Testing
-
-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
-# Run all tests
-ctest --preset debug
-
-# Run only golden-file tests / only smoke tests
-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/
-```
-