if

Using @if. This can be used as a check used in other commands.

foo:
	@if [ -z $(FOO) ]; then \
		echo "FOOOOO must be set; export FOO=<foo>"; exit 10;
	fi

ifneq

Using ifneq. Note lack of indentation.

foo:
ifneq ($(wildcard ./fizz/$(BUZZ)),)
	$(eval NAME=$(shell sh -c "grep ..."))
endif
	...