switch between building ffi and native rust with one Cargo.toml

[jrhea]

Currently, there is a dirty hack to build cdynlib for c, java, dotnet targets and normal build for rust. I literally created a Cargo-ffi.toml and a Cargo-rust.toml and switch between them with symbolic links in the Makefile. I'm sure there is some way to do this with just one Cargo.toml, but idk how and cargo hates me.

@hermanjunge maybe you know how to fix this?