message: cmake: Reverts back to C++17 (C++20 on GCC 10.1 / Ubuntu results in internal compiler error.
diff: @@ -9,7 +9,7 @@ file(WRITE "${CMAKE_BINARY_DIR}/version.txt" "${CONTOUR_VERSION_STRING}")
  project(contour VERSION "${CONTOUR_VERSION}" LANGUAGES CXX)
  # setting defaults
  -set(CMAKE_CXX_STANDARD 20)
  +set(CMAKE_CXX_STANDARD 17)
  set(CMAKE_CXX_STANDARD_REQUIRED ON)
  set(CMAKE_CXX_EXTENSIONS OFF)
  