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/basic/macro.h

@@ -105,6 +105,15 @@
         _Pragma("GCC diagnostic push");                                 \
         _Pragma("GCC diagnostic ignored \"-Wincompatible-pointer-types\"")
 
+#if HAVE_WSTRINGOP_TRUNCATION
+#  define DISABLE_WARNING_STRINGOP_TRUNCATION                           \
+        _Pragma("GCC diagnostic push");                                 \
+        _Pragma("GCC diagnostic ignored \"-Wstringop-truncation\"")
+#else
+#  define DISABLE_WARNING_STRINGOP_TRUNCATION                           \
+        _Pragma("GCC diagnostic push")
+#endif
+
 #define REENABLE_WARNING                                                \
         _Pragma("GCC diagnostic pop")