build: Allow disabling building of wayland libraries

When cross-compiling it may be useful to build only the wayland-scanner
natively.  This patch makes it possible to disable build of the libraries.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>