autotools: fix selecting wincng in cross-builds (and more)

- Fix explicitly selecting WinCNG in autotools cross-builds by moving
  `windows.h` header check before the WinCNG availability check.
  Follow-up to d43b8d9b0b

  Reported-by: Jack L
  Fixes #1186

- Add Linux -> mingw-w64 cross-builds for autotools and CMake. This
  doesn't detect #1186, because that happened when explicitly specifying
  WinCNG via `--with-crypto=wincng`, but not when falling back to WinCNG
  by default.

- autotools: fix to strip suffix from gcc version

  Before this patch we expected `n.n` `-dumpversion` output, but Ubuntu
  may return `n-win32` (also with `-dumpfullversion`). Causing these
  errors and failing to enable picky warnings:
  ```
  ../configure: line 23845: test: : integer expression expected
  ```
  Ref: https://github.com/libssh2/libssh2/actions/runs/6263453828/job/17007893718#step:5:143

  Fix that by stripping any dash-suffix.

  gcc version detection is still half broken because we translate '10'
  to '10.10' because `cut -d. -f2` returns the first word if the
  delimiter missing.

  More possible `-dumpversion` output: `10-posix`, `10-win32`,
  `9.3-posix`, `9.3-win32`, `6`, `9.3.0`, `11`, `11.2`, `11.2.0`
  Ref: https://github.com/mamedev/mame/pull/9767

Closes #1187

.github/workflows/ci.yml

@@ -165,6 +165,43 @@ jobs:
           export OPENSSH_SERVER_IMAGE=ghcr.io/libssh2/ci_tests_openssh_server:$(git rev-parse --short=20 HEAD:tests/openssh_server)
           cd bld && ctest -VV --output-on-failure
 
+  build_linux_cross_mingw64:
+    name: 'linux -> mingw-w64'
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        build: [autotools, cmake]
+    env:
+      TRIPLET: 'x86_64-w64-mingw32'
+    steps:
+      - uses: actions/checkout@v3
+      - name: 'install packages'
+        run: sudo apt-get --quiet 2 --option Dpkg::Use-Pty=0 install mingw-w64
+      - name: 'autotools autoreconf'
+        if: ${{ matrix.build == 'autotools' }}
+        run: autoreconf -fi
+      - name: 'autotools configure'
+        if: ${{ matrix.build == 'autotools' }}
+        run: mkdir bld && cd bld && ../configure --enable-werror --enable-debug --host=${TRIPLET}
+      - name: 'autotools build'
+        if: ${{ matrix.build == 'autotools' }}
+        run: make -C bld -j3
+      - name: 'cmake configure'
+        if: ${{ matrix.build == 'cmake' }}
+        run: |
+          cmake --version
+          cmake -B bld \
+            -DCMAKE_SYSTEM_NAME=Windows \
+            -DCMAKE_C_COMPILER_TARGET=${TRIPLET} \
+            -DCMAKE_C_COMPILER=${TRIPLET}-gcc \
+            -DENABLE_WERROR=ON
+
+      - name: 'cmake build'
+        if: ${{ matrix.build == 'cmake' }}
+        run: cmake --build bld --parallel 3
+
   build_cygwin:
     name: 'cygwin'
     runs-on: windows-latest