cmake: GH1428, Reenable subproject builds
cmake: GH1428, Reenable subproject builds

SCRIPTS dir was defined using CMAKE_SOURCE_DIR which prevented LVL
from building correctly if used as a subproject. Changed to use
CMAKE_CURRENT_SOURCE_DIR.

Change-Id: I201fcfde78b65df80c3c74373a26e4d518770806

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers