Add runtime libloading support

[Xuanwo]

Take https://github.com/KyleMayes/clang-sys/blob/master/src/link.rs as reference. We can load libhdfs during runtime instead.

[SKTT1Ryze]

Honestly, dlopen is not the common way to build a binding crate, just link so/dylib in build.rs during compile time instead.

// build.rs
println!("cargo:rustc-link-lib=dylib=awesome_math");

[Xuanwo]

Thanks for the advice! My intention with this issue is not to completely replace rustc-link-lib via runtime link. Instead, I aim to enable users to build and run applications without linking them to JVM or HDFS, which most users may not have set up in their environment.