Attempt at fixing load-time linking

This time I've made all binaries look for libfluidsynth relative to @loader_path. Previously we used @executable_path — which correctly points to our binary when we are the plugin host (that's why the .app distribution was working) — but would fail more generally (other Audio Plugin Hosts would supply their own @executable_path, which points to their binary, when really we want to point to ours).