minor, fix Makefile if expression.

#==========================

dist: dist-check

build:

	if [ ! python3 -m pytest --version ]; then python3 -m pip install --user pytest; fi

	if ! python3 -m pytest --version; then python3 -m pip install --user pytest; fi

	python3 -m pytest build.py

	./build.py

	sed -i "s/PKG_VERSION/$(VERSION)/g" zero-input.el