Use CMake's Ninja backend

The difference between Make and Ninja for full, non-incremental builds
isn't that much [1,2]; but Ninja is already being used by Meson, so
why not?

[1] https://david.rothlis.net/ninja-benchmark/
[2] http://hamelot.io/programming/make-vs-ninja-performance-comparison/

!386
2 jobs for master in 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #185185
flatpak devel

00:00:09

 
  Deploy
skipped #185186
pages
 
Name Stage Failure
failed
flatpak devel Test
Using docker image sha256:2507a02fbe19aecae28444f64d707695d37eed03de93a4387dafcf374336b831 for registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:3.30 ...
Running on runner-3ef8a5d5-project-1-concurrent-0 via m1-xlarge-x86.gnome.org...
Cloning repository...
Cloning into '/builds/GNOME/nautilus'...
remote: fatal: packfile ./objects/pack/pack-4ede80aa476c4786ff7649bbe4240aa8b0b1eec6.pack cannot be accessed
remote: aborting due to possible repository corruption on the remote side.
fatal: protocol error: bad pack header
/bin/bash: line 74: cd: /builds/GNOME/nautilus: No such file or directory
ERROR: Job failed: exit code 1