OS X: Make deployment of clang lib less fragile.

Ensure that the correct rpath is added, even if we fail to remove
the old one.

Change-Id: I96e37d0e26b04f240300244aa93f434a60013410
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>