Bug 986807 patch 3 - Use preprocessor to avoid duplicating reftest default preference lists.  r=ted

Note that the reordering in Makefile.in is because the addition to
PP_TARGETS must be before including rules.mk.  (Logically, I was moving
_DEST_DIR, _HARNESS_FILES, and the _HARNESS_PP_FILES chunk up, but diff
shows it the other way around.)