Latest meson doesn't work with older python 3.5, which is present on
Ubuntu 16.04. Let's pin in to the latest working version (0.52.1) until
we properly bump all necessary Ubuntu images to 18.04.
See: https://github.com/mesonbuild/meson/issues/6427
Now that https://github.com/systemd/systemd/pull/12542 is merged,
fuzzbuzz.sh should be changed a little bit to make it work
on Azure Pipelines. We can no longer assume that source repositories
are added "automagically" by Travis CI or that PATH is set properly.