Did you know it was possible to export environment variables at the top-level of a Makefile?
Makefile illustrates just that:
export MSG="Hello World!" .PHONY: hello 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.