Use autoconf glue around new my_is_symlink to avoid stat(2) and friends

being used on systems which don't support that interface.

mysys/my_symlink.c

@@ -109,8 +109,12 @@ int my_symlink(const char *content, const char *linkname, myf MyFlags)
 
 int my_is_symlink(const char *filename __attribute__((unused)))
 {
+#if defined(HAVE_REALPATH) && !defined(HAVE_purify) && !defined(HAVE_BROKEN_REALPATH)
   struct stat stat_buff;
   return !lstat(filename, &stat_buff) && S_ISLNK(stat_buff.st_mode);
+#else
+  return 0;
+#endif
 }