shared/utmp-wtmp: silence gcc warning about strncpy truncation

shared/utmp-wtmp: silence gcc warning about strncpy truncation

Unfortunately the warning must be known, or otherwise the pragma generates a
warning or an error. So let's do a meson check for it.

Is it worth doing this to silence the warning? I think so, because apparently
the warning was already emitted by gcc-8.1, and with the recent push in gcc to
catch more such cases, we'll most likely only get more of those.