Add separate headers for system-specific enums, thus reducing coupling. Suggested by Emil Dotchevski.

[SVN r41033]

test/system_error_test.cpp

@@ -42,7 +42,7 @@ namespace
     BOOST_CHECK( ex.code().category() == system_category );
 # ifdef BOOST_WINDOWS_API
     LANGID language_id;
-#   ifndef __MINGW32__
+#   if !defined(__MINGW32__) && !defined(__CYGWIN__)
       language_id = ::GetUserDefaultUILanguage();
 #   else
       language_id = 0x0409; // Assume US English