Linux: Fix build with dbus=no or threads=no

[akien-mga]

No description provided.

[akien-mga]

BTW, I found this while testing an editor build with threads=no, and finding that platform/linuxbsd/freedesktop_at_spi_monitor.h requires threads but doesn't check THREADS_ENABLED. CC @bruvzg

Overall using the editor single-threaded on desktop is not really a use case we want to actively support, but if we can make it build without too much hassle, it's a convenient way to test behavior for the single-threaded web editor on desktop.

[bruvzg]

requires threads but doesn't check THREADS_ENABLED

Same for freedesktop_portal.

[deralmas]

BTW, I found this while testing an editor build with threads=no, and finding that platform/linuxbsd/freedesktop_at_spi_monitor.h requires threads but doesn't check THREADS_ENABLED.

The Wayland backend depends on a thread for its event handling, should it check for that too?

Edit: I think that the X11 backend also handles certain events in a thread.

[bruvzg]

Windows and macOS display servers use threads as well, not sure if this flag is meaningful for anything except Web.

[akien-mga]

Windows and macOS display servers use threads as well, not sure if this flag is meaningful for anything except Web.

It's not, as I mentioned above. But it used to compile and work fine as of #85939, and it's useful occasionally to test how the editor behaves on a single-thread without having to use the Web Console for debugging, which is less convenient than a native app/debugger. It's not meant to be a supported configuration but if it's not hard to keep it working, I see no harm in doing so. For Windows/macOS, I agree we can just ignore it.

I've amended the commit to disable dbus when threads is off for Linux.

[Calinou]

Tested locally, it works as expected. Tested locally, it works as expected. I've used a threads=no editor to open and run some demo projects.