Add CI for mingw-w64 via msys2 (#742)

Credit: Jeroen Ooms

.github/workflows/ci.yml

@@ -101,3 +101,31 @@ jobs:
       - uses: actions/checkout@v2
       - name: Run Fuzzer
         run: GIT_REF=$GITHUB_REF ./ci/ossfuzz.sh
+
+  build_msys2:
+    name: msys2
+    runs-on: windows-latest
+    strategy:
+      matrix:
+        include:
+          - { sys: mingw64, env: x86_64 }
+          - { sys: mingw32, env: i686 }
+          - { sys: ucrt64,  env: ucrt-x86_64 }
+          - { sys: clang64, env: clang-x86_64 }
+      fail-fast: false
+    steps:
+      - uses: actions/checkout@v3
+      - uses: msys2/setup-msys2@v2
+        with:
+          msystem: ${{matrix.sys}}
+          install: >-
+            mingw-w64-${{matrix.env}}-cc
+            mingw-w64-${{matrix.env}}-autotools
+            mingw-w64-${{matrix.env}}-openssl
+      - name: Building libssh2
+        shell: msys2 {0}
+        run: |
+          autoreconf -fi
+          ./configure --enable-debug --enable-static --disable-shared --with-crypto=openssl
+          make
+          make check