I googled "universal crt" and read the msdn article (thanks for this). I now see Microsoft redesigned the cruntime for Visual Studio 2015. This can be necessary because the cruntime is both compiler and platform specific.
Now to try and answer my own question. I downloaded the SFML source and I see there is a directory named extlibs with a subdirectory name libs-msvc. Now there are libs for both 32 and 64 bit Windows. These libraries were compiled with an older version of the cruntime library, so their in-place generation of some functions will be outdated. Hence the errors. You'd need to track down the flac, freetype, jpeg, ogg, openal32, vorbis, vorbisenc, and vorbisfile source and recompile it with VS2015. Then you can build SFML with VS2015.
I am fine using VS2013 for now. I just wanted to understand what was happening. That being VS2015 has been released (it's no longer a release candidate).
