Only create symlinks if they don't already exist

If the second parameter to ln is a directory (or link to one) it creates a symlink inside it (unless the -T) option is specified. Changed the loop so it only creates symlinks if they don't exist to avoid geting symlinks in the symlinked direcdtories linking to the symlinked directory. (Symlinkception!)
When I do that, I get a warning saying that non virtual destructor might cause undefined behaviour. Since no class extends MapLight, it will not be a problem but IMO, we should try to avoid such warnings as much as possible...