message: Fixup usage of CMake variables.
  This CL fixes the VULKAN_HPP_VULKAN_HEADERS_SRC_DIR and VULKAN_HPP_PATH
  usages to use the values of the variables instead of the text strings.
diff: @@ -62,7 +62,7 @@ message("CMAKE_CXX_STANDARD = <${CMAKE_CXX_STANDARD}>")
  if (NOT DEFINED VulkanRegistry_DIR)
  if (DEFINED VULKAN_HPP_VULKAN_HEADERS_SRC_DIR)
  - set(VulkanRegistry_DIR VULKAN_HPP_VULKAN_HEADERS_SRC_DIR)
  + set(VulkanRegistry_DIR ${VULKAN_HPP_VULKAN_HEADERS_SRC_DIR})
  else()
  set(VulkanRegistry_DIR "${CMAKE_CURRENT_SOURCE_DIR}/Vulkan-Headers")
  endif()
  @@ -73,7 +73,7 @@ add_definitions(-DVK_SPEC="${vk_spec}")
  if (NOT DEFINED VulkanHeaders_INCLUDE_DIR)
  if (DEFINED VULKAN_HPP_PATH)
  - set(VulkanHeaders_INCLUDE_DIR VULKAN_HPP_PATH)
  + set(VulkanHeaders_INCLUDE_DIR ${VULKAN_HPP_PATH})
  else()
  set(VulkanHeaders_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
  endif()
  