How can I make a pattern rule dependency optional in a Makefile?

Question

I would like make to reference the timestamp of a dependency if and only if the file already exists. I have a pattern rule like this:

%.pdf: %.sil
          


        

Answer 1

With a sufficiently new version of GNU make you can use:

.SECONDEXPANSION:
%.pdf: %.sil $$(wildcard $$*.lua)
        sile $< -o $@

See the manual section for SECONDEXPANSION targets and the wildcard function.