Move rustc_version dep to version_check

The `rustc_version` dependency is weighty and suffers from
djc/rustc-version-rs#11 which makes it more difficult to change
rustc over time.

Author: alexcrichton

Cargo.toml

@@ -70,7 +70,7 @@ wasm-bindgen = { version = "0.2.12", optional = true }
 average = "0.9.2"
 
 [build-dependencies]
-rustc_version = "0.2"
+version_check = "0.1"
 
 [package.metadata.docs.rs]
 all-features = true

build.rs

@@ -1,14 +1,19 @@
-extern crate rustc_version;
-use rustc_version::{version, Version};
+extern crate version_check;
 
 fn main() {
-    if version().unwrap() >= Version::parse("1.25.0").unwrap() {
-        println!("cargo:rustc-cfg=rust_1_25");
-    }
-    if version().unwrap() >= Version::parse("1.26.0").unwrap() {
+    let version = match version_check::is_min_version("1.25.0") {
+        Some((true, version)) => version,
+        Some((false, _)) | None => return,
+    };
+    let mid = match version.split('.').nth(1).and_then(|s| s.parse::<u32>().ok()) {
+        Some(v) => v,
+        None => return,
+    };
+    println!("cargo:rustc-cfg=rust_1_25");
+    if version.starts_with("1.") && mid >= 26 {
         println!("cargo:rustc-cfg=rust_1_26");
     }
-    if version().unwrap() >= Version::parse("1.27.0").unwrap() {
+    if version.starts_with("1.") && mid >= 27 {
         println!("cargo:rustc-cfg=rust_1_27");
     }
 }