Update of frame work for PHP and support for PHP 5.4.x.

* PHP_VERSION_DEFAULT, PHP_VERSIONS_ACCEPTED, PKG_PHP_VERSION now
  don't accept 5 any more but 52, 53, 54.
  Each value corresponding to PHP 5.2.x, 5.3.x, 5.4.x.
* PHP_PKG_PREFIX might be "php54".
* phpversion.mk defines PHP_BASE_VERS.
* phpversion.mk defines each PHP's exact version for now.

TODO:
php{5,53,54}/Makefile.{common,php} could be arranged to some
redundant codes.

(taca)