Deprecate/warn upon using MD5 with URL_HASH
CMake's URL_HASH accepts MD5 as a hash algorithm:
https://cmake.org/cmake/help/v3.5/module/ExternalData.html
While md5 is popular, it's also well known to be broken. It's continued use should be everywhere discouraged.
It would be nice if CMake warned upon it's use, to get people to switch to SHA256 or better.