I'm not familiar enough with autoconf to edit the configure.ac file, but...

The location in configure.ac is around line 3902

In the configure script, when it checks to see if _res is an extern, there are two problems:

1.  There is no ac_WHATEVER variable to control it and override it.
2.  Because the extern reference to _res is unused, the linker on some systems may elide it, causing an erroneous definition of HAS__RES_EXTERN

The test should be coded in "configure" as follows:

/* ... confdefs.h */
#include <stdio.h>
#if HAVE_SYS_TYPES_H
# include <sys/types.h>
#endif
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
extern struct __res_state _res;

int
main ()
{
struct __res_state *volatile p = &_res;  /* force resolution of _res */ return 0; }