Change MAKE envvar to MK to fix make variable inheritance

Setting MAKE=/path/to/make creates a subtle problem: the Chicken
makefile system expects to pass env vars, such as PREFIX, via
inheritance, to its make subprocesses.  It does NOT set them
explicitly in the submake invocation line.  And it uses $(MAKE)
as the path to submake.

Upshot is if your make script looks like
  make PREFIX=foo $@
and you call it via
  MAKE=mymake
  $MAKE PREFIX=bar ...

then only the toplevel makefile gets PREFIX=bar (make PREFIX=foo PREFIX=bar)
while the subfiles get PREFIX=foo (mymake).