Add build script part to FFI chapter for more clear and smooth learn …

[RoggeOhta]

Fix #345 #122
Make FFI chapter snappy example workable.

[jamischarles]

@RoggeOhta Thank you thank you thank you. I've been banging my head wondering how on earth to reference snappy.

[jamischarles]

FYI: on macOS I didn't need println!("cargo:rustc-link-lib=dylib=stdc++"); this line in the build script.
In fact it caused an error
note: ld: library 'stdc++' not found

[jamischarles]

My working build file:

fn main() {
    // println!("cargo:rustc-link-lib=dylib=stdc++");
    println!("cargo:rustc-link-search=/Users/jac/dev/snappy/build");
}

Didn't realize I needed to ref the build folder