[Feature Request]: build onnxruntime in ort-sys

[Chiichen]

I'm trying to use ort on Android platform. However, it requires me to manually build onnxruntime. For me, the build commands mentioned in #177 works for me. It's quite simple actually. Just run the build.sh with some specific arguments and set the environment variables correctly. I think we can make it as a feature (such as build-from-sourece) to build the libs automatically