How to force g++ to respect #define _POSIX_C_SOURCE 200809L

I need to use strerror_r to translate error numbers into human readable messages compiled with g++ on Linux Debian Bullseye. The man page notes:

int strerror_r(int errnum, char *buf, size_t buflen);
           /* XSI-compliant */

char *strerror_r(int errnum, char *buf, size_t buflen);
           /* GNU-specific */

strerror_r():
   The XSI-compliant version is provided if:
   (_POSIX_C_SOURCE >= 200112L) && !  _GNU_SOURCE
   Otherwise, the GNU-specific version is provided.

We have two different types of the return value: int or char* depending on the version defined by _POSIX_C_SOURCE. I have this small test program:

~$ cat strerror_r.c
#include <string.h>
#include <stdio.h>

// #define _POSIX_C_SOURCE 200112L
// #undef _GNU_SOURCE

#define ERROR_BUFFER_LEN (size_t)256

int main(int argc, char **argv)
{
#if _POSIX_C_SOURCE < 200112L
    char* ret;
#else
    int ret;
#endif
    char errorBuffer[ERROR_BUFFER_LEN];
    int errno;

    errno = 0;
    ret = strerror_r(errno, errorBuffer, ERROR_BUFFER_LEN);
    fprintf(stderr, "Error message by pointer = '%s'\n", ret);
    fprintf(stderr, "Content of errorBuffer =  '%s'\n", errorBuffer);

    return 0;
}

If I compile it with gcc everything is as expected:

$ gcc strerror_r.c && ./a.out; rm a.out
Error message by pointer = '(null)'
Content of errorBuffer =  'Success'

If I compile it with g++ I get this:

$ g++ strerror_r.c && ./a.out; rm a.out
strerror_r.c: In function ‘int main(int, char**)’:
strerror_r.c:24:21: error: invalid conversion from ‘char*’ to ‘int’ [-fpermissive]
   24 |     ret = strerror_r(errno, errorBuffer, ERROR_BUFFER_LEN);
      |           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                     |
      |                     char*
rm: cannot remove 'a.out': No such file or directory

If I try to force the needed version by uncommenting

#define _POSIX_C_SOURCE 200112L
#undef _GNU_SOURCE

I get:

$ g++ strerror_r.c && ./a.out; rm a.out
strerror_r.c:7: warning: "_POSIX_C_SOURCE" redefined
    7 | #define _POSIX_C_SOURCE 200112L
      |
In file included from /usr/include/x86_64-linux-gnu/bits/libc-header-start.h:33,
                 from /usr/include/string.h:26,
                 from strerror_r.c:3:
/usr/include/features.h:281: note: this is the location of the previous definition
  281 | # define _POSIX_C_SOURCE 200809L
      |
strerror_r.c: In function ‘int main(int, char**)’:
strerror_r.c:24:21: error: invalid conversion from ‘char*’ to ‘int’ [-fpermissive]
   24 |     ret = strerror_r(errno, errorBuffer, ERROR_BUFFER_LEN);
      |           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                     |
      |                     char*
rm: cannot remove 'a.out': No such file or directory

What I'm missing here? Why g++ does not compile the default thread save version of strerror_r? I need that version. How can I fix it?

Reference

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