check for preg_replace in install.php

to avoid problems like in http://forum.dokuwiki.org/thread/521

darcs-hash:20070102204857-7ad00-9a352a5503114c2df20eb2aded243c2c9e50abe8.gz

install.php

@@ -427,7 +427,8 @@ function check_functions(){
                          'file file_exists fseek flush filesize ftell fopen '.
                          'glob header ignore_user_abort ini_get mail mkdir '.
                          'ob_start opendir parse_ini_file readfile realpath '.
-                         'rename rmdir serialize session_start unlink usleep');
+                         'rename rmdir serialize session_start unlink usleep'.
+                         'preg_replace');
 
     if (!function_exists('mb_substr')) {
       $funcs[] = 'utf8_encode';