Add build.rs which is used to check the Rust version before building.

build.rs

@@ -0,0 +1,14 @@
+extern crate rustc_version;
+use rustc_version::{Version, version};
+
+fn main() {
+    let version = version().expect("Failed to get rustc version.");
+    if version < Version::parse("1.42.0").unwrap() {
+        eprintln!(
+            "\n\nAt least Rust version 1.42 is required.\n\
+             Version {} is used for building.\n\
+             Build aborted.\n\n",
+             version);
+        panic!();
+    }
+}