Add flag to force POCO to preserve manually #defined _WIN32_WINNT value

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 to _WIN32_WINNT_WINXP, but this setting gets wiped out by the first block of Foundation/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, e.g. NetworkInterface.