Add constant for path to XInclude tolerant RelaxNG

configure.php

@@ -26,6 +26,7 @@
 
 const RNG_SCHEMA_DIR = __DIR__ . DIRECTORY_SEPARATOR . 'docbook' . DIRECTORY_SEPARATOR . 'docbook-v5.2-os' . DIRECTORY_SEPARATOR . 'rng' . DIRECTORY_SEPARATOR;
 const RNG_SCHEMA_FILE = RNG_SCHEMA_DIR . 'docbook.rng';
+const RNG_SCHEMA_XINCLUDE_FILE = RNG_SCHEMA_DIR . 'docbookxi.rng';
 
 function usage() // {{{
 {