Defines:
* XRT_HAVE_[name] is a external dep indicator.
* XRT_DRIVER_[name] if a internal driver is built.
CMake variables:
* BUILD_WITH_[name] is a external dep indicator.
* BUILD_DRIVER_[name] tells if we should build a specific driver.
For enabling Vulkan validation during runtime.
Add CMake option to disable Vulkan validation at build time.
By default the DEBUG extension is enabled but the validation layers
are not when the ENV variable is not set to true.
