Remove more old-compiler compensation.

Remove more old-compiler compensation.

We require an environment with a C99-compatible snprintf(), so we don't
need to work around older implementations.  Make the configuration
process fail if we don't have snprintf() and vsnprintf().

We require at least VS 2015, so we don't have to check for _MSC_VER >=
1400.  Make the build fail if we don't have at least VS 2015.

We apparently do, however, have to use __inline, as the VS 2015
documentation doesn't meaning plain old "inline".  Update a comment.