As discovered earlier, Homebrew gcc is built against a specific Apple
SDK version and doesn't work when matched up with a different version,
e.g. the one advertised as default by the macos runner image.
Before this patch this was resolved with brute force by zapping the
hack-layer gcc component to avoid the bad interference. This worked
for us, but it's fragile, accidental and doesn't translate to
real-world build environments. Thus, impractical.
Avoid this by explicitly selecting the SDK version gcc was built for and
meant to be used with, as shown by `gcc --print-sysroot`.
It assumes that the gcc binaries preinstalled on the runner images
always ship with the SDK version they reference. It also assumes
this works with and without `brew update`.
Also:
- add 4 quick build-only jobs to test all gcc/macos combos.
- list SDKs offered via CommandLineTools.
Suggested-by: Bo Anderson
Ref: https://github.com/Homebrew/homebrew-core/issues/194778#issuecomment-2462764619
Follow-up to c349bd668c#14097Closes#15518
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