lintcheck: add -j <N> option to configure threads.

defaults to 1
-j 0 choses the number of threads automtically (= number of physical cores)

clippy_dev/src/main.rs

@@ -69,6 +69,14 @@ fn get_clap_config<'a>() -> ArgMatches<'a> {
                 .value_name("CRATES-SOURCES-TOML-PATH")
                 .long("crates-toml")
                 .help("set the path for a crates.toml where lintcheck should read the sources from"),
+        )
+        .arg(
+            Arg::with_name("threads")
+                .takes_value(true)
+                .value_name("N")
+                .short("j")
+                .long("jobs")
+                .help("number of threads to use, 0 automatic choice"),
         );
 
     let app = App::new("Clippy developer tooling")