CI: Build also on macOS 15

- Cover ARM with macOS 15 and x86_64 with macOS 13
- Disable the autotools build for now

Undefined symbols for architecture arm64:
Linking libaudcore.dylib...
  "_iconv", referenced from:
Failed to link libaudcore.dylib!
      str_convert(char const*, int, char const*, char const*) in charset.lib.o
  "_iconv_close", referenced from:
      str_convert(char const*, int, char const*, char const*) in charset.lib.o
  "_iconv_open", referenced from:
      str_convert(char const*, int, char const*, char const*) in charset.lib.o
ld: symbol(s) not found for architecture arm64
clang++: error: linker command failed with exit code 1

.github/workflows/c-cpp.yml

@@ -10,8 +10,13 @@ jobs:
   build:
     strategy:
       matrix:
-        os: ['ubuntu-20.04', 'ubuntu-22.04', 'ubuntu-24.04', 'macos-13', 'windows-2022']
-        build-system: ['autotools', 'meson']
+        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04,
+             macos-13, macos-15,
+             windows-2022]
+        build-system: [autotools, meson]
+        exclude:
+          - os: macos-15
+            build-system: autotools
       fail-fast: false
     runs-on: ${{ matrix.os }}