build: always build wayland-scanner

The previous idiom for building a cross-compiled Wayland is to build once for
the build host (with --enable-scanner --disable-libraries) to get a
wayland-scanner binary that can then be used in a cross-compile (with
--disable-scanner).  The problem with this is that the cross wayland is missing
a wayland-scanner binary, which means you then can't do any Wayland development
on the target.

Instead, always build wayland-scanner for the target and change
--enable/disable-scanner to --with/without-host-scanner.  Normal builds use the
default of --without-host-scanner and run the wayland-scanner it just built, and
cross-compiled builds pass --with-host-scanner to use a previously built host
scanner but still get a wayland-scanner to install.

(a theoretically neater solution would be to build two scanners if required (one
to run and one to install), but automake makes this overly complicated)

[daniels: Bikeshedded naming with Ross's OK.]

Signed-off-by: Ross Burton <ross.burton@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>