New warnings can be hard to notice in CI, since CI will just pass in
that case. Meson sometimes uses warnings for important mistakes, e.g.
invalid option.
Let's turn warnings into errors so that we can spot these more easily.
If unspecified, feature options are set to "auto", which means enabled only if
the required dependencies are found. In CI we don't want to miss compilation
errors because a dependency hasn't been found and code isn't built.
Leave FreeBSD out for now because it uses a subproject (haven't found a way to
make auto_features=enabled only apply to the toplevel project).
Manuel Stoeckl
Simon Ser
Kenny Levinsen
Drew DeVault