With the old method of adding build RPATHs, only the libsoup as
a subproject was supported. However, it's possible to use Homebrew for
libsoup too by adding libsoup as a dependency to the target (even
though we don't link to it).
Apple ld will not add a LC_LOAD_DYLIB entry for a library unless there
are symbols that need it because meson passes `-dead_strip_dylibs`, so
we can do this without issue.
As a bonus, this ensures the correct build RPATHs in all cases.
Additionally, we still need to explicitly add an LC_RPATH for the
installed case.
Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/4027
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8403>
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