Instead of having a build-time option to enable/disable xwayland
support, just use the wlroots build config: enable xwayland in
Sway if it was enabled when building wlroots. I don't see any
use-case for disabling xwayland in Sway when enabled in wlroots:
Sway doesn't pull in any additional dependency (just pulls in
dependencies that wlroots already needs). We have a config command
to disable xwayland at runtime anyways.
This makes it so xwayland behaves the same way as other features
such as libinput backend and session support. This also reduces
the build matrix (less combinations of build options).
I think we originally introduced the xwayland option when we didn't
have a good way to figure out the wlroots build config from the
Sway build system.
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).
