gitlab-ci: use old meson version on Ubuntu 16.04 to work with ninja-1.5.1

Meson 0.54.0 requires ninja-1.7 ([1]).

On Ubuntu 16.04, we now would get meson 0.54.0 via pip3, but ninja-1.5.1 via
apt. That doesn't work anymore.

We could install ninja via pip3, but of course, doing that on other
Debian/Ubuntu versions fails due to ... I don't even want to know.

So, instead use an old meson version on Ubuntu 16.04, which is
known to still work with the ninja provided by the packaging system.

We anyway don't want to test the same meson/ninja versions on all our
Ubuntu/Debian images. The point of having different images is to build
with different software versions. If `pip3 install` gives us the same
everywhere, it isn't very useful.

https://mesonbuild.com/Release-notes-for-0-54-0.html#ninja-version-requirement-bumped-to-17

.gitlab-ci.yml

@@ -41,7 +41,8 @@ stages:
     - date '+%Y%m%d-%H%M%S'; dbus-uuidgen --ensure
     - date '+%Y%m%d-%H%M%S'; sed -i 's/^# \(pl_PL.UTF-8 .*\)$/\1/p' /etc/locale.gen ; true
     - date '+%Y%m%d-%H%M%S'; locale-gen pl_PL.UTF-8
-    - date '+%Y%m%d-%H%M%S'; pip3 install meson
+    - date '+%Y%m%d-%H%M%S'; grep -q "VERSION=.16.04.6 LTS" /etc/os-release && pip3 install meson==0.53.2
+    - date '+%Y%m%d-%H%M%S'; grep -q "VERSION=.16.04.6 LTS" /etc/os-release || pip3 install meson
     - date '+%Y%m%d-%H%M%S'; contrib/scripts/nm-ci-install-valgrind-in-ubuntu1604.sh
 
     # iproute2 5.2.0 on debian:sid causes our unit tests to fail.