fix build breakage due to makefile syntax confusion
fix build breakage due to makefile syntax confusion

Consider the following makefile construct:

target: dep0
target: dep1 dep2
  command -o $(@) $(^)

With some versions of make, $(^) will expand to "dep1 dep2", but on
others it will expand to "dep0 dep1 dep2".  The commit ensures that we
no longer rely on the former behavior (nor the latter, for that matter).

dicej/avian,marcinolawski/avian,marcinolawski/avian,bigfatbrowncat/avian-pack.avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,MaartenR/avian,MaartenR/avian,minor-jason/avian,minor-jason/avian,marcinolawski/avian,dicej/avian,getlantern/avian,ucdseniordesign/avian,lwahlmeier/avian,lostdj/avian,badlogic/avian,getlantern/avian,MaartenR/avian,lwahlmeier/avian,badlogic/avian,ucdseniordesign/avian,MaartenR/avian,ucdseniordesign/avian,lwahlmeier/avian,bgould/avian,marcinolawski/avian,badlogic/avian,bgould/avian,dicej/avian,lostdj/avian,bgould/avian,joshuawarner32/avian,minor-jason/avian,joshuawarner32/avian,joshuawarner32/avian,lwahlmeier/avian,getlantern/avian,badlogic/avian,minor-jason/avian,dicej/avian,bgould/avian,bigfatbrowncat/avian-pack.avian,ucdseniordesign/avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,getlantern/avian,joshuawarner32/avian