`DEBUG` env var in build.rs is incorrect if profile debug=0

[ehuss]

The DEBUG environment variable set for a build script currently gets set to true if you have a profile with debug=0. AFAIK, rustc treats debug=0 the same as no debug, so I think it should be false.

Repro:

cargo new --lib a
cd a
cat >> Cargo.toml <<EOL
[profile.dev]
debug = 0
EOL
cat > build.rs <<EOL
fn main() { println!("DEBUG={}", std::env::var("DEBUG").unwrap()); }
EOL
cargo build -vv

I can fix this if you'd like.

[alexcrichton]

Ah yes thanks this'd be good to fix!