Add flag to force POCO to preserve manually #defined target Windows versions

I'm trying compile Poco with backwards compatibility to Windows XP and Server 2003 using the XP toolset in Visual Studio 2013 (see http://msdn.microsoft.com/en-us/library/jj851139.aspx). To do so, I need to manually #define _WIN32_WINNT and NTDDI_VERSION, but these settings get wiped out by Platform_WIN32.h since the VS2013 SDK also contains values for Vista/7/etc. This causes incorrect code to be compiled into classes that depend on the value of _WIN32_WINNT and NTDDI_VERSION, e.g. NetworkInterface.