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.

src/backend/Makefile

@@ -174,8 +174,9 @@ $(top_builddir)/src/include/catalog/schemapg.h: catalog/schemapg.h
 	  $(LN_S) "$$prereqdir/$(notdir $<)" .
 
 $(top_builddir)/src/include/utils/errcodes.h: utils/errcodes.h
-	cd '$(dir $@)' && rm -f $(notdir $@) && \
-	    $(LN_S) "../../../$(subdir)/utils/errcodes.h" .
+	prereqdir=`cd '$(dir $<)' >/dev/null && pwd` && \
+	  cd '$(dir $@)' && rm -f $(notdir $@) && \
+	  $(LN_S) "$$prereqdir/$(notdir $<)" .
 
 $(top_builddir)/src/include/utils/fmgroids.h: utils/fmgroids.h
 	prereqdir=`cd '$(dir $<)' >/dev/null && pwd` && \