Add a GitHub action for cross-compiling to MinGW via CMake.

.github/workflows/c-cpp.yml

@@ -17,3 +17,16 @@ jobs:
       run: ./configure
     - name: make
       run: make
+
+  build-cmake-mingw:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: apt
+      run: sudo apt-get install mingw-w64
+    - name: cmake
+      run: cmake -DCMAKE_TOOLCHAIN_FILE=i686-w64-mingw32.cmake .
+    - name: make
+      run: make