VS2013 introduced a lot of C99 (enough to compile FFmpeg), VS2013 update 2 introduced more C99 (compiling almost x264 and a bit further VLC). This update adds some of the biggest issues, related to printf and scanf formats, and fixes more C99 reported bugs.
From the VC++ blog: "Most of the remaining C99 library features are implemented."
I love to hate Microsoft like the next guy, but they've done a lot on C99 lately.