Use AC_REPLACE_FUNCS

strndup is removed from AC_CHECK_FUNCS since it will be provided if not
found.

Fixes #2610

libi3/strndup.c

@@ -10,8 +10,7 @@
 #include <sys/types.h>
 #include <string.h>
 
-#if defined(__APPLE__)
-
+#ifndef HAVE_strndup
 /*
  * Taken from FreeBSD
  * Returns a pointer to a new string which is a duplicate of the
@@ -30,5 +29,4 @@ char *strndup(const char *str, size_t n) {
     copy[len] = '\0';
     return (copy);
 }
-
 #endif