message: autogen.sh: Default to C++20 when using this script.
diff: @@ -53,6 +53,7 @@ exec cmake "${ROOTDIR}" \
  -DCMAKE_CXX_COMPILER="${CXX}" \
  -DPEDANTIC_COMPILER=ON \
  -DPEDANTIC_COMPILER_WERROR=ON \
  + -DCMAKE_CXX_STANDARD=20 \
  ${EXTRA_CMAKE_FLAGS} \
  -B "${BUILD_DIR}" \
  -GNinja
  