Parsing of Variables on Windows doesn't respect windows conventions

Our use of shlex in posix mode, unconditionally, means that we need to specify paths in an odd format.

We should, on Windows, default the shlex object to non-posix mode. However, we should also offer an option to expressly select a lexing mode so that the user can select one that makes sense for odd environments (like cygwin), where the windows behavior may not be correct.