message: Add a missing link in the book for `jobserver`. I don't know what the original
  intended link destination was, so I chose to link to the GNU Make documentation.
diff: @@ -126,6 +126,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
  [profile]: reference/manifest.html#the-profile-sections
  [configuration]: https://doc.rust-lang.org/reference/attributes.html#conditional-compilation
  [clang]: http://clang.llvm.org/docs/CrossCompilation.html#target-triple
  +[jobserver]: https://www.gnu.org/software/make/manual/html_node/Job-Slots.html
  ### Environment variables Cargo sets for 3rd party subcommands
  