WinRT: build fix for Visual C++ 2013 Update 4

Visual C++ 2013 Update 4 re-introduced the Sleep() function to WinRT apps (for
code that targets Windows 8.1 and Windows Phone 8.1).  This led to a build
error, as SDL was defining it's own Sleep() function (to make up for the lack
of a public Sleep() function).  The fix makes sure that SDL's custom Sleep()
function is only used when Windows' Sleep() is not available.

Many thanks go out to Sergiu Marian Gaina for the fix!
WinRT: made SDL_GetPlatform() return "WinRT" and not 'Unknown'

Thanks to Eric Wing for the heads-up on this!