Merge from gnu/dist:

Make the null printf format string warning enabled only if -pedantic is
supplied.  This allows printf format attributes on err/warn.  A fix to handle
this properly is in the works for the next major gcc release.

gnu/dist/toolchain/gcc/c-common.c

@@ -1398,7 +1398,8 @@ check_format_info (info, params)
 
   if (integer_zerop (format_tree))
     {
-      warning ("null format string");
+      if (pedantic)
+        warning ("null format string");
       return;
     }
   if (TREE_CODE (format_tree) != ADDR_EXPR)