soup: Ensure that libsoup RPATHs are added with Homebrew too

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 RPATHs in all cases.

Additionally, we still need to explicitly add an LC_RPATH for the installed case.

Fixes #4027 (closed)

@thewildtree could you please try this? It works for me with both subproject libsoup and homebrew libsoup.