bootstrap: update and enable the LLVM version-check

While the `config.toml.example` comments say "we automatically check the
version by default," we actually didn't.  That check was badly out of
date, only allowing 3.5, 3.6, or 3.7.  This it now updated to the new
3.9 minimum requirement, and truly enabled by default.

config.toml.example

@@ -35,7 +35,7 @@
 # If an external LLVM root is specified, we automatically check the version by
 # default to make sure it's within the range that we're expecting, but setting
 # this flag will indicate that this version check should not be done.
-#version-check = false
+#version-check = true
 
 # Link libstdc++ statically into the librustc_llvm instead of relying on a
 # dynamic version to be available.

src/bootstrap/config.rs

@@ -299,6 +299,7 @@ impl Config {
         let mut config = Config::default();
         config.llvm_enabled = true;
         config.llvm_optimize = true;
+        config.llvm_version_check = true;
         config.use_jemalloc = true;
         config.backtrace = true;
         config.rust_optimize = true;

src/bootstrap/native.rs

@@ -259,11 +259,14 @@ fn check_llvm_version(build: &Build, llvm_config: &Path) {
 
     let mut cmd = Command::new(llvm_config);
     let version = output(cmd.arg("--version"));
-    if version.starts_with("3.5") || version.starts_with("3.6") ||
-       version.starts_with("3.7") {
-        return
+    let mut parts = version.split('.').take(2)
+        .filter_map(|s| s.parse::<u32>().ok());
+    if let (Some(major), Some(minor)) = (parts.next(), parts.next()) {
+        if major > 3 || (major == 3 && minor >= 9) {
+            return
+        }
     }
-    panic!("\n\nbad LLVM version: {}, need >=3.5\n\n", version)
+    panic!("\n\nbad LLVM version: {}, need >=3.9\n\n", version)
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]