Set ninja=true by default

Ninja substantially improves LLVM build time. On a 96-way system, using
Make took 248s, and using Ninja took 161s, a 35% improvement.

We already require a variety of tools to build Rust. If someone wants to
build without Ninja (for instance, to minimize the set of packages
required to bootstrap a new target), they can easily set `ninja=false`
in `config.toml`.  Our defaults should help people build Rust (and LLVM)
faster, to speed up development.

README.md

@@ -36,6 +36,7 @@ by running `./x.py --help` or reading the [rustc dev guide][rustcguidebuild].
    * `python` 3 or 2.7
    * GNU `make` 3.81 or later
    * `cmake` 3.4.3 or later
+   * `ninja`
    * `curl`
    * `git`
    * `ssl` which comes in `libssl-dev` or `openssl-devel`