Meson 0.48 or newer is required. On Linux and macOS you can get meson through your package manager or using:
$ pip3 install --user meson
This will install meson into ~/.local/bin which may or may not be included automatically in your PATH by default.
You should get ninja using your package manager or download the official release and put the ninja binary in your PATH.
You can get all GStreamer built running:
meson build/ ninja -C build/
Create two complete branches in the same pipeline
./gst-n-launch -b "videotestsrc ! autovideosink" -b "videotestsrc num-buffers=100 ! videoconvert ! autovidosink"