CI: Add a GitHub Action to build on Windows

This build in any commits and pull requests.

.github/workflows/build.yml

@@ -0,0 +1,39 @@
+name: build-win
+
+on:
+  push:
+      branches: [ master ]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: windows-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include: [
+          { msystem: MINGW64, arch: x86_64 },
+          # currently fail
+          #{ msystem: MINGW32, arch: i686   }
+        ]
+    steps:
+
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Build
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: ${{ matrix.msystem }}
+          install: mingw-w64-${{ matrix.arch }}-gcc mingw-w64-${{ matrix.arch }}-cmake mingw-w64-${{ matrix.arch }}-make mingw-w64-${{ matrix.arch }}-capstone mingw-w64-${{ matrix.arch }}-glfw mingw-w64-${{ matrix.arch }}-glm mingw-w64-${{ matrix.arch }}-file mingw-w64-${{ matrix.arch }}-llvm mingw-w64-${{ matrix.arch }}-nlohmann-json mingw-w64-${{ matrix.arch }}-openssl mingw-w64-${{ matrix.arch }}-polly mingw-w64-${{ matrix.arch }}-python
+          update: true
+
+      - name: Build
+        shell: msys2 {0}
+        run: |
+          mkdir build
+          cd build
+          cmake -G "MinGW Makefiles" ..
+          mingw32-make -j