cmake: Add VULKAN_HEADERS_INSTALL option

Allows add_subdirectory users to install vulkan-headers

.github/workflows/linux.yml

@@ -19,6 +19,9 @@ on:
     pull_request:
         branches:
             - main
+
+env:
+  CMAKE_GENERATOR: Ninja
             
 permissions:
     contents: read
@@ -32,7 +35,7 @@ jobs:
         with:
           cmakeVersion: 3.17.0
       - name: Configure Vulkan-Headers
-        run: cmake -S . -B build -G "Ninja"
+        run: cmake -S . -B build
       - name: Install Vulkan-Headers
         run: cmake --install build --prefix ${{ github.workspace }}/build/install
       - name: Test Vulkan-Headers find_package support
@@ -46,7 +49,7 @@ jobs:
         with:
           cmakeVersion: 3.25.0
       - name: Configure Vulkan-Headers
-        run: cmake -S . -B build -D BUILD_TESTS=ON --log-level=DEBUG -G "Ninja"
+        run: cmake -S . -B build -D BUILD_TESTS=ON --log-level=DEBUG
       - name: Build Vulkan-Headers Tests
         run: cmake --build build
 
@@ -58,6 +61,16 @@ jobs:
         with:
           cmakeVersion: 3.15.0
       - name: Configure Vulkan-Headers
-        run: cmake -S . -B build/ -G "Ninja" --loglevel=DEBUG
+        run: cmake -S . -B build/ --loglevel=DEBUG
       - name: Install Vulkan-Headers
         run: cmake --install build/ --prefix build/install
+
+  ubuntu-cmake-add_subdirectory:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: lukka/get-cmake@latest
+      - name: Test add_subdirectory support
+        run: cmake -S tests/add_subdirectory -B tests/add_subdirectory/build
+      - name: Build
+        run: cmake --build tests/add_subdirectory/build

CMakeLists.txt

@@ -57,7 +57,10 @@ if (PROJECT_IS_TOP_LEVEL)
     if (BUILD_TESTS)
         add_subdirectory(tests)
     endif()
+endif()
 
+option(VULKAN_HEADERS_INSTALL "Install Vulkan Headers" ${PROJECT_IS_TOP_LEVEL})
+if (VULKAN_HEADERS_INSTALL)
     include(GNUInstallDirs)
     include(CMakePackageConfigHelpers)
 

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
+)