Introduces PHP_VERSION_ID for faster checking

[stodorovic]

Since PHP 5.2.7, the PHPVERSIONID constant is available. Also, WP core has fallback for this constant in wp-includes/random_compat/random.php. So, it doesn't exist only in very old installations of WP on very old PHP versions. It's very rare case and I've added fallback for this case.

Example of usage:

if ( PHP_VERSION_ID >= 50300 ) {
    clearstatcache( true, $wp_cache_config_file );
}