# $(if) is even less useful than test(1).
undefined = $(subst undefined,,$(flavor $(1)))
defined = $(if $(call undefined,$(1)),defined,)

FOO = $(if $(call defined,$(__FOO)),,$(eval __FOO := foo))$(__FOO)

lazyval = $(if $(call defined,__$(1)),,$(eval __$(1) := $$(value 2)))$(__$(1))
lazydef = $(eval $(1) = $(call lazyval,$(1),$(value 2)))

$(call lazydef,BAR,$$(shell sleep 1; echo bar))

all: foo bar

foo:
	@echo FOO is $(call defined,FOO)
	@echo FOO\'s value is $(FOO)

bar:
	@echo BAR is $(call defined,BAR)
	@echo BAR\'s value is $(BAR)