C Makefile: How to build environment variable into executable

I’m trying to get an environment variable passed from the shell into an executable when it gets compiled, and be able to access that variable. For example, say I wanted to build the time something was compiled into the application when it gets built so I can see when the executable was built. How do I structure the Makefile and C program to do that?

Example C program:

#include <stdio.h>
#define variable 2

void main(){
printf("Variable: %d\n", variable);
}

Example Makefile:

CC=gcc
CFLAGS=-I
BUILD_TIME=$(date)
example: example.c
        $(CC) -o example example.c

How can these two files be modified to make the BUILD_TIME variable available to the C file?

>Solution :

You can use the -D option to create preprocessor variables on the command line.

CC=gcc
CFLAGS=-I
BUILD_TIME=`date`
example: example.c
        $(CC) -D "BUILD_TIME=\"$(BUILD_TIME)\"" -o example example.c

#include <stdio.h>

int main()
{
    printf("build time = %s\n", BUILD_TIME);
    return 0;
}