devel/wayland: Make this cross-compile.

For cross-compilation, it depends on a native build of itself, for
the wayland-scanner tool.  This, in turn, requires a patch to the
meson files to allow it to be passed as a parameter (rather than
discovered through cmake or pkg-config, which are generally limited
to references into the cross-destdir).

(If someone can figure out how to spell `./configure
ac_cv_foo_path=...' in meson to obviate the need for that patch,
please do so!  I searched around in the manual and I couldn't find
any way to cause something like override_find_program to happen from
a command-line argument or environment variable.)

(riastradh)