Update comment related to C99

One could do more work here to eliminate the Windows difference
described in the comment, but that can be a separate project.  The
purpose of this change is to update comments that might confusingly
indicate that C99 is not required.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/170308e6-a7a3-4484-87b2-f960bb564afa%40eisentraut.org

src/bin/pg_dump/dumputils.h

@@ -25,9 +25,10 @@
  * We don't print the timezone on Windows, because the names are long and
  * localized, which means they may contain characters in various random
  * encodings; this has been seen to cause encoding errors when reading the
- * dump script.  Think not to get around that by using %z, because
- * (1) %z is not portable to pre-C99 systems, and
- * (2) %z doesn't actually act differently from %Z on Windows anyway.
+ * dump script.  One could now possibly get around that by using %z, but %z
+ * was previously not portable to pre-C99 systems, and also previously %z
+ * didn't actually act differently from %Z on Windows.  But of these problems
+ * might be obsolete now.
  */
 #ifndef WIN32
 #define PGDUMP_STRFTIME_FMT  "%Y-%m-%d %H:%M:%S %Z"