ci: raise error when Meson generates warnings

ci: raise error when Meson generates warnings

Meson prints warnings in several cases, like for example when using a
feature supported by the current version of Meson, but not yet supported
by the minimum required version as declared by the project. These
warnings will not cause the setup to fail by default, which makes it
quite easy to miss them.

Improve this by passing `--fatal-meson-warnings` to `meson setup` so
that our CI jobs will fail on warnings.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>