Merge the parts that has things to do with stdint.h and inttypes.h with
the !_MSC_VER portions, and add initial support for Visual Studio 2015,
which added support for C99 snprintf() and vsnprintf().

Not too sure about the !_MSC_VER for C99 snprintf() and vsnprintf(), but
since this file is mainly for Visual Studio builds, anyways...