keg_relocate: add HOMEBREW_PREFIX/lib to rpaths

[carlocab]

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...on non-/usr/local prefixes.

This allows dlopen to just work on Homebrew-installed (and linked)
libraries. We don't need to do this for /usr/local/lib because this is
already in dyld's default search path.

We already do this manually via setting LDFLAGS for some formulae
(e.g. Python, LuaJIT, etc).

I'll likely wait until we've removed HOMEBREW_RELOCATABLE_INSTALL_NAMES gating before merging this.

[carlocab]

An alternative implementation would be to add the appropriate -rpath flags to superenv/LDFLAGS.

I think the approach here is better because it guarantees that HOMEBREW_PREFIX/lib is added to rpaths last, which is important because we don't want to inadvertently pick up, say, a linked library of a different version for a keg-only formula.

[MikeMcQuaid]

I think the approach here is better because it guarantees that HOMEBREW_PREFIX/lib is added to rpaths last, which is important because we don't want to inadvertently pick up, say, a linked library of a different version for a keg-only formula.

Agreed 👍🏻

[github-actions]

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