Now, ar/lib rules can list static libraries or other .d files. These (as well as
the target library) are written to a file called $lib.d. link/exe rules can list
these .d files, causing them to be flattened into an RSP file and appear on the
command line as @$rsp.
The rule to generate the dependency list files depends on the libraries they
contain, so they also act as a stamp file for the dependencies. This gives us
dependency management for free.
