Makefile: download libbpf submodule when not found

[geyslan]

Making contribution easier.

[rafaeldtinoco]

This way an offline build would fail, wouldn't it ? Sometimes we may need to rsync the git tree to an offline node to test building, for example. Maybe to check if directory is empty and then try to update it ?

Not sure about the submodule managing by Makefile also, @grantseltzer what is your take on this ?

[geyslan]

Maybe to check if directory is empty and then try to update it ?

The rule already does something like that since LIBBPF_SRC = $(abspath ./libbpf/src). So the rule only runs if ./libbpf/src doesn't exist.

$(LIBBPF_SRC):
ifeq ($(wildcard $@), )
	echo "INFO: updating submodule 'libbpf'"
	$(GIT) submodule update --init --recursive
endif

[rafaeldtinoco]

ifeq ($(wildcard $@), )

Sorry, missed that. Okay then, I'm ok with this.

[grantseltzer]

LGTM! Thanks so much @geyslan !