message: Add message for appleclang versions that don't support std::filesystem
diff: @@ -3,6 +3,12 @@ project(vcpkg C CXX)
  if(CMAKE_COMPILER_IS_GNUXX OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
  set(GCC 1)
  +elseif(CMAKE_CXX_COMPILER_ID MATCHES "AppleClang")
  + if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS_EQUAL 9.0.0)
  + message(FATAL_ERROR "Apple clang versions 9 and below do not have support for the C++ Filesystem TS. Please install gcc6 or newer from homebrew (brew install gcc6).")
  + else()
  + set(CLANG 1)
  + endif()
  elseif(CMAKE_CXX_COMPILER_ID MATCHES "[Cc]lang")
  set(CLANG 1)
  elseif(MSVC)
  