Discovered while reading through lapce's codebase, this variable collects all MakeFile paths we have included so far in the current makefile (including self).
This is used by several people to autogenerate a help action in makefiles, here's the one used in lapce:
help: ## Print this help message
@grep -E '^[a-zA-Z._-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'