Prevent multiple definition of ASIO_ERROR_CATEGORY_NOEXCEPT with newest MSVC.

asio/include/asio/detail/config.hpp

diff --git a/asio/include/asio/detail/config.hpp b/asio/include/asio/detail/config.hpp
index 183756a..c85f9a2 100644
--- a/asio/include/asio/detail/config.hpp
+++ b/asio/include/asio/detail/config.hpp
@@ -294,8 +294,7 @@
 #     define ASIO_ERROR_CATEGORY_NOEXCEPT noexcept(true)
 #   endif // defined(__GXX_EXPERIMENTAL_CXX0X__)
 #  endif // ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)) || (__GNUC__ > 4)
-# endif // defined(__GNUC__)
-# if defined(ASIO_MSVC)
+# elif defined(ASIO_MSVC)
 #  if (_MSC_VER >= 1900)
 #   define ASIO_ERROR_CATEGORY_NOEXCEPT noexcept(true)
 #  endif // (_MSC_VER >= 1900)