nss uses hard-coded @executable_path (which is wrong, consider e.g. the case of
the URE uno executable), so patch it to use @_..._OOO instead (and no need to
set --prefix), and pass the resulting libs through macosx-change-install-names
(which requires the generated libs to be writable).

Change-Id: I0f04533f0f0581ee7b9dfd8929b8629c0842cc1b