define other-rule
.PHONY: other-rule
other-rule:
        @ echo other-rule
endef

define other-rule2
.PHONY: other-rule2
other-rule:
        @ echo other-rule2
endef

define rule_test
.PHONY: rule-test
rule-test: other-rule
ifeq ($$(strip $$(VAR)),)
$$(eval $$(call other-rule))
else
$$(eval $$(call other-rule2))
endif
endef

$(eval $(rule_test))