runnable.rb: ignore broken symlink error on Windows [ci skip]

tool/mkrunnable.rb

@@ -45,6 +45,10 @@ end
 
 def ln_safe(src, dest)
   ln_sf(src, dest)
+rescue Errno::ENOENT
+  # Windows disallows to create broken symboic links, probably because
+  # it is a kind of reparse points.
+  raise if File.exist?(src)
 end
 
 alias ln_dir_safe ln_safe