`ninja` is only used in Windows builds on CI.

[eddyb]

AFAICT this is the only thing that adds --enable-ninja, and it's Windows-only:

rust/src/ci/scripts/install-ninja.sh

Lines 9 to 16 in b9ac291

	if isWindows; then
	mkdir ninja
	curl -o ninja.zip "${MIRRORS_BASE}/2017-03-15-ninja-win.zip"
	7z x -oninja ninja.zip
	rm ninja.zip
	ciCommandSetEnv "RUST_CONFIGURE_ARGS" "${RUST_CONFIGURE_ARGS} --enable-ninja"
	ciCommandAddPath "$(pwd)/ninja"
	fi

We also autodetect ninja and use it, but only for MSVC specifically (#43767 / #44084).

On top of being (supposedly) faster than relying on make, IME ninja also has less log spew when building LLVM (but it's unclear if it would help on CI, see #70468).

cc @rust-lang/infra