cmake: add CPack support for creating binary archives

.github/workflows/riscos.yml

@@ -44,6 +44,9 @@ jobs:
         echo "SDL3_DIR=${{ github.workspace }}/prefix_cmake" >> $GITHUB_ENV
         cmake --install build/
         ( cd ${{ github.workspace }}/prefix_cmake; find ) | LC_ALL=C sort -u
+    - name: Package (CPack)
+      run: |
+        cmake --build build/ --config Release --target package
     - name: Verify CMake configuration files
       run: |
         cmake -S cmake/test -B cmake_config_build -G Ninja \
@@ -52,3 +55,8 @@ jobs:
           -DCMAKE_BUILD_TYPE=Release \
           ${{ matrix.platform.test_args }}
         cmake --build cmake_config_build --verbose
+    - uses: actions/upload-artifact@v3
+      with:
+        if-no-files-found: error
+        name: SDL-riscos
+        path: build/dist/SDL3*