main: Display git commit hash with the '--version' option

Add a build-script to propagate the git commit hash to other crates at
compile time through environment variables, and display the hash along
with the '--version' option.

Fixes #729

Signed-off-by: Bo Chen <chen.bo@intel.com>

build.rs

@@ -0,0 +1,22 @@
+// Copyright © 2020 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+use std::process::Command;
+
+fn main() {
+    let git_out = Command::new("git")
+        .args(&["describe", "--dirty"])
+        .output()
+        .expect("Expect to get git describe output");
+
+    // This println!() has a special behavior, as it will set the environment
+    // variable BUILT_VERSION, so that it can be reused from the binary.
+    // Particularly, this is used from src/main.rs to display the exact
+    // version.
+    println!(
+        "cargo:rustc-env=BUILT_VERSION={}",
+        String::from_utf8(git_out.stdout).unwrap()
+    );
+}