message: Improve message for appleclang
diff: @@ -4,8 +4,12 @@ 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).")
  + if(NOT VCPKG_ALLOW_APPLE_CLANG)
  + message(FATAL_ERROR
  +"Building the vcpkg tool requires support for the C++ Filesystem TS.
  +Apple clang versions 9 and below do not have support for it.
  +Please install gcc6 or newer from homebrew (brew install gcc6).
  +If you would like to try anyway, set VCPKG_ALLOW_APPLE_CLANG.")
  else()
  set(CLANG 1)
  endif()
  