Be consistent when building backlinks. With that said though, you might want to linkbuild to all of them at least a bit because I would imagine that google sees that as natural, but for the bulk of your linkbuilding you should do it to the address in your address bar when you go to the site. That's my rule of thumb anyway.

www[.] site[.] com and site[.] com are treated as two different sites. www is like a subdomain. While making backlinks be consistent with either one.

I got it now. Thank you!

doesn't have any difference if you're redirecting www to non-www by default. And i would recommend to build a backlinks to the url which is showing up in the browser address bar when you visit your website.

And in the Google webmaster tools, you may change the appearance in the search results with or without www