Definition by its authors
Ninja is a small build system with a focus on speed. It differs from other build systems in two major respects: it is designed to have its input files generated by a higher-level build system, and it is designed to run builds as fast as possible. (https://ninja-build.org/)
Ninja instead of make
Ninja can be used instead of make, in particular as a different generator in cmake.
Usage in O2
Ninja will be used if you have ninja available in your PATH. And on Mac the easiest/fastest way to achieve this is
brew install ninja.
Once you have ninja in your PATH, if you do not want to use it when
alibuilding O2 or QC, you will need to export DISABLE_NINJA=1 beforehand.
Note that this only works for development packages.