Overriding Makefile variables

override.mk

test1: CFLAGS ?= -Wall -Wextra

test2: CFLAGS += -Wall -Wextra

test3: override CFLAGS += -Wall -Wextra

test4: override CFLAGS := -Wall -Wextra $(CFLAGS)

test1 test2 test3 test4:
	@echo "$@: $(CFLAGS)"

.PHONY: test1 test2 test3 test4

$ for test in test{1,2,3,4}; do make -f override.mk $test; done
test1: -Wall -Wextra
test2: -Wall -Wextra
test3: -Wall -Wextra
test4: -Wall -Wextra

$ for test in test{1,2,3,4}; do make CFLAGS="-g" -f override.mk $test; done
test1: -g
test2: -g
test3: -g -Wall -Wextra
test4: -Wall -Wextra -g

$ for test in test{1,2,3,4}; do CFLAGS="-g" make -f override.mk $test; done
test1: -g
test2: -g -Wall -Wextra
test3: -g -Wall -Wextra
test4: -Wall -Wextra -g