gcc -I. -I/usr/pkg/include -g -O2 -flto -c ./util.c
./util.c: In function 'xmallocarray':
./util.c:267:24: warning: implicit declaration of function 'reallocarray' [-Wimplicit-function-declaration]
         void *result = reallocarray(NULL, num, size);
                        ^
./util.c:267:24: warning: initialization makes pointer from integer without a cast [-Wint-conversion]

#define _OPENBSD_SOURCE
#include <stdlib.h>
void *
reallocarray(void *ptr, size_t nmemb, size_t size);

$ grep -IR reallocarray util.c
        void *result = reallocarray(NULL, num, size);
                log_msg(LOG_ERR, "reallocarray failed: %s", strerror(errno));

The reallocarray() function first appeared in OpenBSD 5.6.
reallocarray() was redesigned in NetBSD 8 as reallocarr(3).  For compati-
bility reasons it's available since NetBSD 8 in the _OPENBSD_SOURCE
namespace.

This function is provided for source compatibility with OpenBSD and its
use is discouraged in preference to reallocarr(3).

$ gcc -dM -E - </dev/null | grep -i -E 'linux|bsd|gnu|i*86|x86|amd' | sort
#define __GNUC_MINOR__ 5
#define __GNUC_PATCHLEVEL__ 0
#define __GNUC_STDC_INLINE__ 1
#define __GNUC__ 5
#define __NetBSD__ 1
#define __amd64 1
#define __amd64__ 1
#define __x86_64 1
#define __x86_64__ 1