Don't re-compile on every cargo run (#4019)

- Add new crate `substrate-build-script-utils` to unify the code of
`node`, `node-template` and `polkadot-node`.
- The `node-cli` build script needs to search upwards for the
`.git/HEAD` file to find it.

substrate/core/utils/build-script-utils/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "substrate-build-script-utils"
+version = "2.0.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+[dependencies]

substrate/core/utils/build-script-utils/src/lib.rs

@@ -0,0 +1,44 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Crate with utility functions for `build.rs` scripts.
+
+use std::{env, path::PathBuf};
+
+/// Make sure the calling `build.rs` script is rerun when `.git/HEAD` changed.
+///
+/// The file is searched from the `CARGO_MANIFEST_DIR` upwards. If the file can not be found,
+/// a warning is generated.
+pub fn rerun_if_git_head_changed() {
+	let mut manifest_dir = PathBuf::from(
+		env::var("CARGO_MANIFEST_DIR").expect("`CARGO_MANIFEST_DIR` is always set by cargo.")
+	);
+	let manifest_dir_copy = manifest_dir.clone();
+
+	while manifest_dir.parent().is_some() {
+		if manifest_dir.join(".git/HEAD").exists() {
+			println!("cargo:rerun-if-changed={}", manifest_dir.join(".git/HEAD").display());
+			return
+		}
+
+		manifest_dir.pop();
+	}
+
+	println!(
+		"cargo:warning=Could not find `.git/HEAD` searching from `{}` upwards!",
+		manifest_dir_copy.display(),
+	);
+}