Makefile shouldn't unthinkingly retrieve robot.jar

[swartik]

Our IT department is hypersensitive about JARs and makes downloading them difficult. I have to go through contortions to run the make command. I request the Makefile be rewritten to check whether build/lib/robot.jar already exists, and to not download it if it does.

If you agree this request is worthwhile, I'll rewrite Makefile on my fork and submit a pull request.

[BrendaBraitling]

This seems like a sound risk management process that has no impact on users.

[tmprd]

I agree that adding extra security precautions related to this are worthwhile.
However, Make was designed exactly for this use case (though I think it's documentation is confusing):
https://www.gnu.org/software/make/manual/make.html#Rule-Example
If you run make build/lib/robot.jar ideally you should get the message: 'build/lib/robot.jar' is up to date.

But I see the unexpected behavior you mean. We should be able to avoid it another way, so I'll comment on your PR.
I recommend this (related docs):

ROBOT_FILE := $(config.LIBRARY_DIR)/robot.jar
$(ROBOT_FILE): | $(config.LIBRARY_DIR)
	curl -L -o $@ https://github.com/ontodev/robot/releases/download/v1.8.4/robot.jar
	chmod +x $@