message: append CMAKE_CXX_FLAGS instead of overwriting it, or else we end up ignoring extra flags asked for by the user
diff: @@ -9,13 +9,13 @@ endif()
  message(STATUS "Build type: ${CMAKE_BUILD_TYPE}")
  if (MSVC)
  - set(CMAKE_CXX_FLAGS "/GL /EHsc /W2")
  + set(CMAKE_CXX_FLAGS "/GL /EHsc /W2 ${CMAKE_CXX_FLAGS}")
  set(CMAKE_CXX_FLAGS_DEBUG "/Od /Zi")
  set(CMAKE_CXX_FLAGS_RELEASE "/O2 /Oi /Oy /Zc:inline")
  set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "/O2 /Oi /Zi")
  set(CMAKE_EXE_LINKER_FLAGS "/LTCG:INCREMENTAL ${CMAKE_EXE_LINKER_FLAGS}")
  else()
  - set(CMAKE_CXX_FLAGS "-std=c++17 -Wall -march=native -fPIC")
  + set(CMAKE_CXX_FLAGS "-std=c++17 -Wall -march=native -fPIC ${CMAKE_CXX_FLAGS}")
  set(CMAKE_CXX_FLAGS_DEBUG "-Werror -g -Og")
  set(CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
  set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELEASE} -g")
  