configure: Require C99 return value from vsnprintf()
Actually we can do this to all branches, including even S2_6. It's not dropping support for any systems, but just causing them to use fallback vsnprintf() implementation, that should work correctly (it doesn't at the moment, but #45629 is about fixing that)
Related to #45629 about fc_vsnprintf() return value.
Change the "working vsnprintf()" configure check not to consider C89 return value (-1) in case of truncated output acceptable, but require C99 return value (what the lenght would have been if there was no truncation)