[wwwdocs] Mention flexible array type and mangling change

[Martin Sebor]

>> Actually, there is one other thing that might be wort mentioning
>> about flexible array members.
>>
>> The type and mangling of flexible array members has changed.  While
>> in GCC 5 and prior the type of a flexible array member is an array
>> of zero elements (a GCC extension), in 6 it is that of an array of
>> an unspecified bound (i.e., T[] as opposed to T[0]).  This is
>> a silent ABI change with no -fabi-version/-Wabi option.
>
> Aha.  I think you're better-suited to document this than I am.

Sure.  Here's the added text.  Please let me know if it's good to
check in.

Martin


Index: htdocs/gcc-6/porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
retrieving revision 1.12
diff -u -r1.12 porting_to.html
--- htdocs/gcc-6/porting_to.html	11 Feb 2016 18:56:15 -0000	1.12
+++ htdocs/gcc-6/porting_to.html	11 Feb 2016 21:15:30 -0000
@@ -315,6 +315,15 @@
  };
  </code></pre>

+<p>
+Finally, the type and mangling of flexible array members has changed
+from previous releases.  While in GCC 5 and prior the type of a flexible
+array member is an array of zero elements (a GCC extension), in GCC 6 it
+is that of an array of an unspecified bound (i.e., <tt>T[]</tt> as opposed
+to <tt>T[0]</tt>).  This is a silent ABI change with no corresponding
+<tt>-fabi-version</tt> or <tt>-Wabi</tt> option to disable or warn about.
+</p>
+
  <h2>-Wmisleading-indentation</h2>
  <p>
  A new warning <code>-Wmisleading-indentation</code> was added