Just to clarify for others reading this post, that option will tell R to install the package only for the architecture currently in use (i.e. x32 or x64), this usually happens when you install R for both architectures but you only have a compiler for one of them installed in your system, and you are trying to compile packages from source. To avoid this problem, only install R for the architecture you are actually going to use or install the proper compiling tools for both of them.