+#undef _AC_ERRNO_UNKNOWN
+#define _AC_ERRNO_UNKNOWN "unknown error"
+
+#ifdef HAVE_SYS_ERRLIST
+ /* this is thread safe */
+# define STRERROR(e) ( (e) > -1 && (e) < sys_nerr \
+ ? sys_errlist[(e)] : _AC_ERRNO_UNKNOWN )
+
+#elif defined( HAVE_STRERROR )
+ /* this may not be thread safe */
+ /* and, yes, some implementations of strerror may return NULL */
+# define STRERROR(e) ( strerror(e) \
+ ? strerror(e) : _AC_ERRNO_UNKNOWN )
+
+#else
+ /* this is thread safe */
+# define STRERROR(e) ( _AC_ERRNO_UNKNOWN )