Fix linking error under Win 7 - do not import MinCore_Downlevel.lib if target is Win 7 or earlier.
Fix linking error under Win 7 - do not import MinCore_Downlevel.lib if target is Win 7 or earlier.

Applied in PR3243 to 2.4 branch.

apavlenko/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv