[WIP] Accessing each build script's OUT_DIR

[namanlp]

This PR is continuation of my work on GSoC Project : Build Script Delegation

What does this PR try to resolve?

Through this PR, I want the user to be able to access each build script's OUT_DIR.

How to test and review this PR?

There is a feature gate multiple-build-scripts that can be passed via cargo-features in Cargo.toml. So, you have to add

cargo-features = ["multiple-build-scripts"]

Preferably on the top of the Cargo.toml and use nightly toolchain to use the feature

Then, you can access the OUT_DIR of given build script by accessing it like a compile time environment variable, for example:

const BUILD1_OUT_DIR: &'static str = env!("BUILD1_OUT_DIR");
const BUILD2_OUT_DIR: &'static str = env!("BUILD2_OUT_DIR");

fn main() {
    println!("{}", BUILD1_OUT_DIR);
    println!("{}", BUILD2_OUT_DIR);
}