Automatically set temporary file path based on operating system

Basically, it's annoying to have to manually set the environment variable yourself. This manual setting also requires extra clutter in the bash profile

For example: https://stackoverflow.com/questions/2842078/how-do-i-detect-os-x-in-my-vimrc-file-so-certain-configurations-will-only-appl https://stackoverflow.com/questions/3466166/how-to-check-if-running-in-cygwin-mac-or-linux