Bunch of improvements to the `check-environment` rule:
- it was listed as the first target in `Makefile`, causing it to be run
when you just use `make`. It's now moved down to where it is first used.
- Call with `$(MAKE) -s` so that there is less logging noise.
- Use `@echo` so that there is less logging noise.
- Change the example to set the right environment variable (it had a
trailing underscore).
