Windows install issue from file

When attempting to install on Windows from a downloaded version of this repository the use of INSTALL_opts=c("--no-multiarch") does not appear to be valid option.

This seems to limit the ability to install the file in this way in systems that do not have access to git.

This is coming from the fact that there is no rkdb/src/w32 folder in the github repository. Once this has been added then we can omit this limitation.