Makefile

all: x

x: x1.o x2.o

x1.c

#include <stdio.h>

int foo();

static int A;

int main(int argc, char **argv)
{
	A = 1;
	printf("A: %d\n", A);
	printf("foo(): %d\n", foo());
	return 0;
}

x2.c

static int A;

int foo()
{
	A = 2;
	return A;
}