Add configure option to disable scanner compilation

When cross-compiling Wayland, wayland-scanner should be picked up from
the host system instead of compiling and trying to run e.g. ARM
wayland-scanner on X86.

This patch adds --disable-scanner option for disabling the scanner
from the build and using existing wayland-scanner instead (from PATH)
which should help in most cross-compilation systems.

By default everything works as before.

Signed-off-by: Jani Uusi-Rantala <jani.uusi-rantala@nokia.com>