fix some absolute paths in an in-place build
Makefile: make `PKGS' not sticky by default, add `again' target

More generally, `LINK_MODE' controls how the `PKGS' value is saved
or restored: "--save" saves the value (the default), "--restore"
uses a previously saved value (if any), and "" disables saving or
restoring.

The `again' target recurs to `in-place' with `LINK_MODE=--restore'.