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.

src/shared/utmp-wtmp.c

@@ -209,7 +209,7 @@ int utmp_put_init_process(const char *id, pid_t pid, pid_t sid, const char *line
         copy_suffix(store.ut_id, sizeof(store.ut_id), id);
 
         if (line)
-                strncpy(store.ut_line, basename(line), sizeof(store.ut_line));
+                strncpy_exact(store.ut_line, line, sizeof(store.ut_line));
 
         r = write_entry_both(&store);
         if (r < 0)