I've just tried on Windows with IIS + PHP5 and it worked as expected. It shouldn't make a difference with Apache or Linux, and I couldn't find anything on the web about it, but maybe.
Could you just do a basic test to make sure the name is not causing an issue - just put these two files in a folder and run index.php.
<?php include "L'ornement.php"; ?>
<?php print "I have been included!"; ?>
If this works it is probably looking in the wrong place - e.g. is L'ornement.php included via another include in a different directory?