Things changed in the Ninja setup-hook:
- Respect installFlags
- Automatically add checkPhase (can be disabled with dontUseNinjaCheck
  in the same way as dontUseNinjaBuild and dontUseNinjaInstall). Tests
  are only run when "ninja test" exists.
- Error in build phase when build.ninja is missing. We don’t have a
  way to fall back to other build methods, so it’s best to be very
  clear when we aren’t able to build with ninja
- Set -l flag to 1 when enableParallelBuilding is disabled

This is now handled in the ninja setup hook.

(cherry picked from commit f5e0997)

This is now handled in ninja.

These are already in the generated makefiles!

These don’t have much use in ninja, so easiest to leave them out.

This can be set unconditionally to $NIX_BUILD_CORES according to
@orivej.

We should always use this.

Add check phase to ninja setup hook