message: Allow setting vulkan paths.
  This CL adds VULKAN_HPP_VULKAN_HEADERS_SRC_DIR and VULKAN_HPP_PATH
  variables to the CMakeLists file.
diff: @@ -36,13 +36,21 @@ else(MSVC)
  add_compile_options(-Wall)
  endif(MSVC)
  -file(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/Vulkan-Headers/registry/vk.xml vk_spec)
  +if (NOT DEFINED VULKAN_HPP_VULKAN_HEADERS_SRC_DIR)
  + set(VULKAN_HPP_VULKAN_HEADERS_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}/Vulkan-Headers")
  +endif()
  +file(TO_NATIVE_PATH ${VULKAN_HPP_VULKAN_HEADERS_SRC_DIR}/registry/vk.xml vk_spec)
  string(REPLACE "\\" "\\\\" vk_spec ${vk_spec})
  add_definitions(-DVK_SPEC="${vk_spec}")
  -file(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/vulkan/vulkan.hpp vulkan_hpp)
  +if (NOT DEFINED VULKAN_HPP_PATH)
  + set(VULKAN_HPP_PATH "${CMAKE_CURRENT_SOURCE_DIR}/vulkan")
  +endif()
  +file(TO_NATIVE_PATH ${VULKAN_HPP_PATH}/vulkan.hpp vulkan_hpp)
  string(REPLACE "\\" "\\\\" vulkan_hpp ${vulkan_hpp})
  add_definitions(-DVULKAN_HPP_FILE="${vulkan_hpp}")
  +include_directories(${VULKAN_HPP_PATH})
  +
  set(HEADERS
  VulkanHppGenerator.hpp
  