When linking in --make we check the modification time of the
executable against the modification time of the object files, and only
re-link if any object file is newer.  However, we should also check
the modification times of packages, since the recompilation checker
also tracks dependencies in packages.  

In a GHC build this means that if you recompile stage2 and don't
manage to change any fingerpints, we won't recompile Main but we'll
still re-link it.