Export versioned targets for CMake

Generate VulkanHeadersConfig.cmake and version file and install them.
To get the version information the header file vulkan_core.h is parsed.

In client code one can use `find_package(VulkanHeaders)` and get the
targets `Vulkan::Headers` and `Vulkan::Registry`.

The following additional files are generated and installed by CMake:
- VulkanHeadersConfig.cmake:        searched for by find_package(VulkanHeaders)
- VulkanHeadersConfigVersion.cmake: provides version numbers
- VulkanHeadersTargets.cmake:       CMake target definitions used by config file

cmake/Config.cmake.in

@@ -0,0 +1,25 @@
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/@TARGETS_EXPORT_NAME@.cmake")
+check_required_components("@PROJECT_NAME@")
+
+# ALIAS for imported target requires CMake >= 3.11:
+# - https://cmake.org/cmake/help/latest/release/3.11.html#other
+if(NOT CMAKE_VERSION VERSION_LESS 3.11)
+    if(NOT TARGET Vulkan::Headers)
+        set_target_properties(
+            Vulkan::Vulkan-Headers
+            PROPERTIES
+            IMPORTED_GLOBAL True
+        )
+        add_library(Vulkan::Headers ALIAS Vulkan::Vulkan-Headers)
+      endif()
+      if(NOT TARGET Vulkan::Registry)
+        set_target_properties(
+            Vulkan::Vulkan-Registry
+            PROPERTIES
+            IMPORTED_GLOBAL True
+        )
+        add_library(Vulkan::Registry ALIAS Vulkan::Vulkan-Registry)
+    endif()
+endif()