package/wayland: build a host variant

package/wayland: build a host variant

Starting with 1.4.0, weston requires wayland-scanner to build its DT.
So far, we got away with just hand-building wayland-scanner, and made
that available in HOST_DIR/usr/bin

But now, weston also requires wayland's DTD, which we did not take care of
in our dirty little hack.

So, the best solution is to just build a host-variant of wayland, so
we get both wayland-scanner, and the DTD.

But then, we do not build the target wayland-scanner (he, wayland tries
to run it at build time...). So it is a matter of just tweaking a little
bit the host .pc file (which weston uses to find wayland-scanner, sigh...)
and fake it as a target .pc file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>