There's a -MT flag which does what the sed line does. From the gcc man page:

  Change the target of the rule emitted by
  dependency generation... An -MT option will
  set the target to be exactly the string you
  specify...

So in your example one might do something like

  -MT "$@ $(basename $@).d"

which would output

  main.o main.d : main.c defs.h

for the main.o target.