Is $(uname_S) defined here at this point with only the above two
includes?  Don't you need to include ../config.mak.uname as well?

The top-level Makefile does this:

        include config.mak.uname
        -include config.mak.autogen
        -include config.mak

and so should this one, I think, in exactly the same order.

Ah, yes. I have updated the include sequence in contrib/Makefile to:
  include ../config.mak.uname
  -include ../config.mak.autogen
  -include ../config.mak