ICU-21458 icu4c support for macOS @rpath

[evanmiller]

On Darwin:

--enable-rpath incorrectly uses an absolute path rather than an @rpath prefix.

--disable-rpath incorrectly uses a relative path rather than an absolute path. It also uses $(PKGDATA_TRAILING_SPACE) unnecessarily.

See downstream MacPorts discussion here: https://github.com/macports/macports-ports/pull/18696

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-21458
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

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[markusicu]

@garywade

[srl295]

@evanmiller makes sense, thanks for fixing this very longstanding issue!