Add macos-15 CI job

[ahans]

gcc on arm64 macOS does not support sanitizers yet, so we only build with Apple Clang there.

[ryanplusplus]

On macOS gcc is usually an alias for clang -- do you know if gcc is really gcc on macOS runners?

[ahans]

On macOS gcc is usually an alias for clang -- do you know if gcc is really gcc on macOS runners?

You're right, it is not. For gcc there are aliases such as gcc-12, see here. So things would probably work with gcc as well, but would do the exact same thing as the clang version. That seems wasteful. Configuring this via the matrix doesn't seem to work. I think I will just add an if to skip the macOS/gcc configuration.

[ahans]

Turns out matrix has an exclude key. I'm happy with this now.

[ryanplusplus]

Thanks!