Windows install issue from file

[cmccarthy1]

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.

[kimtang]

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.