CRAN-able / better build process

[Paxanator]

In order to get on CRAN we need to make sure the following is done (some of this is sprucing up the build process too)

• Make a configure.win and cleanup.win that does the same thing on windows
• Allow local developers to opt out of pulling and installing librdkafka if they so choose, via an env var or something else. Update the CONTRIBUTING.md Development section with information
• Figure out some best effort way of checking if the library is already installed in a default system path so a user doesn't need to compile all of the library. This doesn't need to be flawless but cover the obvious case of brew install or apt-get etc.
• Ensure that no stray artifacts are being left behind from the build process.
• Minimize path assumptions in Makevars and other places