cmake: Add VULKAN_HEADERS_INSTALL option

Allows add_subdirectory users to install vulkan-headers

tests/add_subdirectory/CMakeLists.txt

@@ -0,0 +1,26 @@
+cmake_minimum_required(VERSION 3.14.2)
+
+project(TEST_VULKAN_HEADERS_ADD_SUBDIRECTORY_SUPPORT LANGUAGES C)
+
+add_subdirectory(../../ ${CMAKE_CURRENT_BINARY_DIR}/headers)
+
+if (NOT TARGET Vulkan::Headers)
+    message(FATAL_ERROR "Vulkan::Headers target not defined")
+endif()
+
+# By default installation for a subproject should be disabled.
+# This makes it easier to consume for most projects.
+# Consuming the vulkan-headers via add_subdirectory and installing
+# them is the more niche use case.
+if (VULKAN_HEADERS_INSTALL)
+    message(FATAL_ERROR "VULKAN_HEADERS_INSTALL should be OFF!")
+endif()
+
+add_library(foobar STATIC)
+
+target_link_libraries(foobar PRIVATE Vulkan::Headers)
+
+target_sources(foobar PRIVATE
+    ../vk_icd.c
+    ../vk_layer.c
+)