Add GitHub Actions build script.

.github/workflows/build.yml

@@ -0,0 +1,21 @@
+name: build
+on: [push]
+jobs:
+  build:
+    runs-on: windows-latest
+    defaults:
+      run:
+        shell: msys2 {0}
+    steps:
+    - uses: actions/checkout@v4
+    - uses: msys2/setup-msys2@v2
+      with:
+        msystem: mingw64
+        install: mingw-w64-x86_64-arm-none-eabi-toolchain
+    - uses: jwlawson/[email protected]
+    - name: Configure
+      run: |
+        cmake -Bbuild .
+    - name: Build
+      run: |
+        cmake --build build --config Debug --target all