meson: Do not warn when a windowing system is not found

Merged Nirbheek Chauhan requested to merge nirbheek/gst-plugins-bad:fatal-meson-warnings into master

Error out when the vulkan option is enabled, and just print a message() otherwise. This is more correct and also allows us to pass --fatal-meson-warnings more reliably.

Edited by Nirbheek Chauhan

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