Ensure minimum rustc version in a build script. r=nalexander (#326)

Printing out failure to meet rustc version helps users during
setup with a helpful message if using an older rustc.
Rust version checking from http://stackoverflow.com/a/36607492.

Cargo.toml

@@ -8,6 +8,10 @@ authors = [
 ]
 name = "mentat"
 version = "0.4.0"
+build = "build/version.rs"
+
+[build-dependencies]
+rustc_version = "0.1.7"
 
 [dependencies]
 clap = "2.19.3"

build/version.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+extern crate rustc_version;
+
+use std::io::{self, Write};
+use std::process::exit;
+use rustc_version::version_matches;
+
+/// MIN_VERSION should be changed when there's a new minimum version of rustc required
+/// to build the project.
+static MIN_VERSION: &'static str  = ">= 1.15.1";
+
+fn main() {
+    if !version_matches(MIN_VERSION) {
+        writeln!(&mut io::stderr(), "Mentat requires rustc {}", MIN_VERSION).unwrap();
+        exit(1);
+    }
+}