How to pass compiler flags to gcc 11 so that it can accept weak variable definitions?

Question

I've got C headers that make variable declarations without "extern", aka weak variable definitions. Compiling that code, gcc 11.2.0 MinGW gives a link error of multiple definitions. gcc 9.4.0 wsl compiles the code normally.

I guess newer version of gcc treats weak variable definitions differently. For some reasons, I cannot add "extern" in the headers. How can I compile the code by providing some compiler flags to gcc 11?

Following is a example which gcc 11 rejects:

//a.h
#ifndef A_H
#define A_H
int a;
#endif

//a.c
#include "a.h"
#include <stdio.h>
int main(){
    printf("a=%d\n",a);
}

//b.c
#include "a.h"
int a=1;