Select C++20 support on the command line

Author: andreasbuhr

.github/workflows/cmake.yml

@@ -91,6 +91,7 @@ jobs:
       run: |
         cmake $GITHUB_WORKSPACE \
           -DCMAKE_BUILD_TYPE=$BUILD_TYPE \
+          -DCMAKE_CXX_STANDARD=20 \
           -DCMAKE_CXX_FLAGS=${{ matrix.config.cxx_flags }} \
           -DCMAKE_EXE_LINKER_FLAGS=${{ matrix.config.exe_linker_flags }} \
           -DCMAKE_VERBOSE_MAKEFILE=ON