Add github CI action for Windows

Hardcode TARGETOS to make it work.

.github/workflows/windows.yml

@@ -0,0 +1,25 @@
+name: windows
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: windows-latest
+    defaults:
+      run:
+        shell: msys2 {0}
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: msys2/setup-msys2@v2
+      with:
+        msystem: MINGW64
+        update: false
+        install: git make mingw-w64-x86_64-toolchain mingw-w64-x86_64-zlib
+    - name: make
+      run: make TARGETOS=windows32