ci: add macOS as a build target

.github/workflows/main.yml

@@ -4,7 +4,7 @@ jobs:
   build:
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest]
+        os: [ubuntu-latest, windows-latest, macos-latest]
     runs-on: ${{matrix.os}}
     steps:
       - uses: actions/checkout@v2
@@ -23,6 +23,10 @@ jobs:
         run: |
             vcpkg integrate install
             vcpkg install sdl2:x64-windows
+      - name: prepare-macos
+        if: runner.os == 'macOS'
+        run: |
+            brew install sdl2
       - name: build 
         run: zig build -Drelease-safe
   lint: