message: Add CMakeList target to run generator
  This CL adds the VULKAN_HPP_RUN_GENERATOR option which will run the
  vulkan hpp generator from CMake if enabled.
diff: @@ -89,6 +89,19 @@ set_property(TARGET VulkanHppGenerator PROPERTY CXX_STANDARD 11)
  target_include_directories(VulkanHppGenerator PRIVATE ${VULKAN_HPP_TINYXML2_SRC_DIR})
  +option (VULKAN_HPP_RUN_GENERATOR "Run the HPP generator" OFF)
  +if (VULKAN_HPP_RUN_GENERATOR)
  + add_custom_command(
  + COMMAND VulkanHppGenerator
  + OUTPUT "${vulkan_hpp}"
  + WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
  + COMMENT "run VulkanHppGenerator"
  + DEPENDS VulkanHppGenerator "${vk_spec}")
  +
  + add_custom_target(build_vulkan_hpp ALL
  + DEPENDS "${vulkan_hpp}" "${vk_spec}")
  +endif()
  +
  option (SAMPLES_BUILD OFF)
  if (SAMPLES_BUILD)
  add_subdirectory(glfw)
  