[bootstrap] Improve the error message when ninja is not found to link to installation instructions.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

src/bootstrap/lib.rs

@@ -1494,8 +1494,12 @@ impl Build {
             {
                 eprintln!(
                     "
-Couldn't find required command: ninja
-You should install ninja, or set `ninja=false` in config.toml in the `[llvm]` section.
+Couldn't find required command: ninja (or ninja-build)
+
+You should install ninja as described at
+<https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages>,
+or set `download-ci-llvm = true` in the `[llvm]` section of `config.toml`
+to download LLVM rather than building it.
 "
                 );
                 std::process::exit(1);