Non-file dependencies in a Makefile

Makefile

# 'gem list -i foo' will print 'true' if a gem is already installed. 'false' otherwise

GEMS=clamp rake

# From the GEMS list, generate a list of make targets
TARGETS=$(foreach gem,$(GEMS),gem-$(shell gem list -i $(gem))-$(gem))
build: $(TARGETS)
build:
	@echo "=> OK to build"

.PHONY: gem-false-%
gem-false-%: GEM=$(@:gem-false-%=%)
gem-false-%:
	@echo Installing "$(GEM)" 
	gem install $(GEM)

.PHONY: gem-true-%
gem-true-%: GEM=$(@:gem-true-%=%)
gem-true-%:
	@echo "Found $(GEM)"