Fix symlink for errcodes.h so it works in VPATH builds from tarballs.

Fix symlink for errcodes.h so it works in VPATH builds from tarballs.

backend/Makefile was treating errcodes.h as a header always generated
during build, but actually it's a header provided in tarballs.  Hence,
must use the absolute-symlink recipe, not the relative-symlink one.
Per bug #6072 from Hartmut Raschick.