Fix duplicate LC_RPATH error on MacOS

Fix duplicate LC_RPATH error on MacOS

After the update to MacOS 15.4, the dynamic loader dyld treats duplicated LC_RPATH as an error.
The `FLAGS` variable already contains `LDFLAGS`, thus using both `FLAGS` and `LDFLAGS`
duplicates all `LDFLAGS`, including `-Wl,rpath` parameters.

The duplicate LC_RPATH causes this kind of errors:

```
dyld[29361]: Library not loaded: @loader_path/../lib/libzstd.1.dylib
      Referenced from: <7131C877-3CF0-33AC-AA05-257BA4FDD770> /Users/foobar/...
      Reason: tried: '/Users/foobar/..../lib/libzstd.1.dylib' (duplicate LC_RPATH '/usr/mypath.../lib')
```

Closes https://github.com/facebook/zstd/issues/4369

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>