meson: Add option to control building wayland-scanner
Wayland requires a binary, wayland-scanner, to be run during the build process. For any configuration other than native builds (including cross compiling and even 32-bit x86 builds on an x86-64 build machine) Wayland's build process builds and uses its own wayland-scanner.
For any builds using a cross file, wayland-scanner is built for the host machine and therefore cannot be executed during the build of the Wayland libraries. Instead builds using a cross file must execute the build machine's wayland-scanner (typically /usr/bin/wayland-scanner).
As such, to build Wayland's libraries for a non-native ABI a package manager must build and install /usr/bin/wayland-scanner first. But then the build for the native ABI then rebuilds wayland-scanner itself and doesn't use the system's, and worse, wants to install its own, which conflicts with the /usr/bin/wayland-scanner already installed!
So, add the -Dscanner=... option to control whether to install wayland-scanner.