I certainly agree with @poupou that PCL's are the way to go (if possible). I would recommend James Montemagno's app on github. He just created this for channel9 using VS 2013. I just created a cross-platform app based off of this and it worked out well. I would only use the file-linking for the platform specific implementations with compiler directives. This can be seen in his ServiceRegistrar class in said app.
This SO answer actually explains how to get the 2012 Project Linker to work with 2013 if you still want to go that route and gives a link to the extension.
Also, you may want to check out MvvmCross. It is open-source, has a large user-base and following, and really helps with maximum code re-use. Best part, it uses PCLs and all of it's features (plugins) are available via nuget.
I would recommend reading the article @poupou posted, watching James' channel9 videos on his github page, and (if you want to check out mvvmcross) watch @slodge's N+1 videos on mvvmcross.