wayland: Keep protocol XML files in-tree.
Now you don't need the latest Wayland installed to build with
newer protocols supported, as they'll build correctly; even if
your system can't use them, we can make intelligent decisions
at runtime about what's available on the current machine anyhow.
This also simplifies some logic and possible failure cases in
the configure and CMake scripts.
Fixes Bugzilla #4207.
