Did you know it was possible to export environment variables at the top-level of a Makefile?

The following Makefile illustrates just that:

export MSG="Hello World!"

.PHONY: hello
	@echo $(MSG)

And the output:

$ make hello
Hello World!

It's also possible to override the environment variable at runtime:

make -e MSG="Hello Venus!" hello
Hello Venus!

Read more about GNU Make Environment Variables in 6.10 Variables from the Environment.