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`

config.toml.example

@@ -49,10 +49,8 @@
 # dynamic version to be available.
 #static-libstdcpp = false
 
-# Tell the LLVM build system to use Ninja instead of the platform default for
-# the generated build system. This can sometimes be faster than make, for
-# example.
-#ninja = false
+# Whether to use Ninja to build LLVM. This runs much faster than make.
+#ninja = true
 
 # LLVM targets to build support for.
 # Note: this is NOT related to Rust compilation targets. However, as Rust is

src/bootstrap/config.rs

@@ -450,6 +450,7 @@ impl Config {
     pub fn default_opts() -> Config {
         let mut config = Config::default();
         config.llvm_optimize = true;
+        config.ninja = true;
         config.llvm_version_check = true;
         config.backtrace = true;
         config.rust_optimize = true;