asio/detail/config.hpp defines _WIN32_WINNT even under WinCE

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Boost.Asio defines _WIN32_WINNT macro even under WinCE environment (_WIN32_WCE == 0x500, Visual Studio 2008).

# if !defined(_WIN32_WINNT) && !defined(_WIN32_WINDOWS)
#  if defined(_MSC_VER) || defined(__BORLANDC__)
#   pragma message( \
  "Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately. For example:\n"\
  "- add -D_WIN32_WINNT=0x0501 to the compiler command line; or\n"\
  "- add _WIN32_WINNT=0x0501 to your project's Preprocessor Definitions.\n"\
  "Assuming _WIN32_WINNT=0x0501 (i.e. Windows XP target).")
#  else // defined(_MSC_VER) || defined(__BORLANDC__)
#   warning Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately.
#   warning For example, add -D_WIN32_WINNT=0x0501 to the compiler command line.
#   warning Assuming _WIN32_WINNT=0x0501 (i.e. Windows XP target).
#  endif // defined(_MSC_VER) || defined(__BORLANDC__)
#  define _WIN32_WINNT 0x0501
# endif // !defined(_WIN32_WINNT) && !defined(_WIN32_WINDOWS)

Maybe it would better to add some UNDER_CE check here?