Install SDL2 through homebrew on macOS CI

.github/workflows/ci.yml

@@ -17,6 +17,9 @@ jobs:
               triplet: x64-osx
       steps:
       - uses: actions/checkout@v2
+      - name: Download SDL2 through Homebrew
+        if: matrix.os == 'macos-latest'
+        run: brew install sdl2
       - name: Download LunarG key
         if: matrix.os == 'ubuntu-latest'
         run: wget -qO - https://packages.lunarg.com/lunarg-signing-key-pub.asc | sudo apt-key add -

CMakeLists.txt

@@ -90,8 +90,13 @@ if(${CMAKE_SYSTEM_NAME} STREQUAL "Emscripten")
 endif()
 
 if(ENABLE_VULKAN)
-    find_package(Vulkan REQUIRED)
-    list(APPEND SHADER_LANGUAGES "spv")
+    find_package(Vulkan)
+    if(NOT TARGET Vulkan::Vulkan)
+        message("Vulkan not found!")
+        set(ENABLE_VULKAN FALSE)
+    else()
+        list(APPEND SHADER_LANGUAGES "spv")
+    endif()
 endif()
 
 if(ENABLE_DX12)