Build path should be canonicalized

[stevefan1999-personal]

#200

This will still be a relative path if the out dir itself is still relative

Given

    let dst = Config::new("../")
        .out_dir("./")
        .build();

Then

cmake-rs/src/lib.rs

Lines 555 to 559 in 94da9de

	let dst = self
	.out_dir
	.clone()
	.unwrap_or_else(|| PathBuf::from(getenv_unwrap("OUT_DIR")));
	let build = dst.join("build");

You will notice build variable is still relative. Thus #200 does not work as intended.