main.c

#include <stdio.h>

int main(void)
{
	int i;
#ifdef FLAG
	i = 1;
#else
	i = 3;
#endif
	printf(" i = %d \n", i);
	return 0;
}

Makefile

CC=gcc

input_flag=

ifdef input_flag
CFLAGS= -D $(input_flag)
endif

SRC= main.c

.PHONY:all clean  

ifdef input_flag
all:
	$(CC) $(CFLAGS) $(SRC) -o main
else
all:
	$(CC) $(CFLAGS) $(SRC) -o main
	@echo "do all"
endif

clean:
	rm *~ *.o -f
 

執行：

snail@ubuntu:4.makefile-if$ make input_flag=FLAG
gcc -D FLAG main.c -o main
snail@ubuntu:4.makefile-if$ ./main 
 i = 1 
snail@ubuntu:4.makefile-if$ make 
gcc  main.c -o main
do all
snail@ubuntu:4.makefile-if$ ./main 
 i = 3 
snail@ubuntu:4.makefile-if$