php/Makefile.am: Fix up SWIG generated C++ wrapper code so that
subclassing of wrapped classes in PHP works with PHP 5.4 and later.
(Fixes #652, reported by Sebastian Gottfried)
php/smoketest.php: Re-enable all tests under PHP 5.4 and later,
which also serves as a regression test for this issue.

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18284 6b2f1b92-a4f3-0310-943f-de3cbe41748b