fix bug #5384 -- define DLL_EXPORT in DLL builds and adjust begin_code.h

include/begin_code.h

@@ -51,14 +51,8 @@
 
 /* Some compilers use a special export keyword */
 #ifndef DECLSPEC
-# if defined(__WIN32__) || defined(__WINRT__)
-#  ifdef __BORLANDC__
-#   ifdef BUILD_SDL
-#    define DECLSPEC
-#   else
-#    define DECLSPEC    __declspec(dllimport)
-#   endif
-#  elif defined(_DLL)
+# if defined(__WIN32__) || defined(__WINRT__) || defined(__CYGWIN__)
+#  ifdef DLL_EXPORT
 #   define DECLSPEC __declspec(dllexport)
 #  else
 #   define DECLSPEC