extend/os/mac/keg_relocate: remove RPATHs rooted in build directory

[carlocab]

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Keeping dangling RPATHs is a security risk, and is bad for build
reproducibility.

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I'm tempted to remove all RPATHs referencing non-existent directories, but I don't know if that will break anything.

Note that delegating delete_rpath doesn't work here. See Homebrew/ruby-macho#355.

[BrewTestBot]

Review period will end on 2021-05-14 at 23:46:37 UTC.

[carlocab]

Re deleting RPATHs that reference non-existent directories: I'd revisit it when we're happy with the current relocation code. I don't want to change too many things at once here.

[BrewTestBot]

Review period ended.