cmake: Use share instead of DATADIR for CMake config files

See the following discussion for context:
https://discourse.cmake.org/t/what-should-the-destination-be-for-a-header-only-librarys-cmake-config-file/8473/3

TLDR: `DATADIR` can be `share` but it doesn't have to be. It can
be changed by the user.

CMakeLists.txt

@@ -68,12 +68,10 @@ if (PROJECT_IS_TOP_LEVEL)
 
  set_target_properties(Vulkan-Headers PROPERTIES EXPORT_NAME "Headers")
 
- set(cmake_install_dir "${CMAKE_INSTALL_DATADIR}/cmake/VulkanHeaders")
-
  install(TARGETS Vulkan-Headers EXPORT VulkanHeadersConfig INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
- install(EXPORT VulkanHeadersConfig NAMESPACE "Vulkan::" DESTINATION "${cmake_install_dir}")
+ install(EXPORT VulkanHeadersConfig NAMESPACE "Vulkan::" DESTINATION "share/cmake/VulkanHeaders")
 
  set(version_config "${CMAKE_CURRENT_BINARY_DIR}/generated/VulkanHeadersConfigVersion.cmake")
  write_basic_package_version_file("${version_config}" COMPATIBILITY SameMajorVersion ARCH_INDEPENDENT)
- install(FILES "${version_config}" DESTINATION "${cmake_install_dir}")
+ install(FILES "${version_config}" DESTINATION "share/cmake/VulkanHeaders")
 endif()