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!
since the window system doesn't do it for us like other platforms.
This prevents sticky keys and missed keys when going in and out
of focus, for example Alt would appear to stick if switching away
from an SDL app with Alt-Tab and had to be pressed again.
CR: Sam
