Allow override of wayland-scanner executable

Allow override of wayland-scanner executable

When cross-compiling, if the environment variables
PKG_CONFIG_SYSROOT_DIR and PKG_CONFIG_LIBDIR are set to the target
sysroot, the wayland_scanner executable won't be found and the
compilation will fail as the xdg-shell header cannot be generated.
Therefore, allow overriding the wayland-scanner executable so that an
alternative executable can be provided with the variable
WAYLAND_SCANNER.

VK-GL-CTS issue: 4139
Components: Framework

Change-Id: I7848a353e93dd07638afadada9c3e0b96ffb5406