Create lib -> lib64 symlink if lib doesn't exist and lib64 does

The convention in slapos is to install libraries in lib/, but on some systems libraries might get installed to lib64/. The current workaround for this is to add a post-install with:

# if lib doesn't exist, then create a symlink 'lib' pointing to lib64
post-install =
  cd ${:location}
  [ -d lib ] || ln -s lib64 lib

(as in libcap)

In my work on NixOS, libraries almost always got installed in lib64/, this fix removes the need to patch every library in slapos. Which I did in this commit for example.