Description:
------------
SimpleXMLElement is essentially a Countable (it matches the signature of the 
interface), but doesn't register as implementing it.

The attached (untested) patch should register SimpleXMLElement as a Countable if 
SPL is also being built. It does not cover the case where someone builds SPL as a 
shared extension (does anyone do that?).

Test script:
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<?php
$x = new SimpleXMLElement('<?xml version="1.0"?><a><b/><c/></a>');
var_dump(count($x), $x instanceof Countable);


Expected result:
----------------
int(2)
bool(true)


Actual result:
--------------
int(2)
bool(false)