error with specifying target

Hello there,

I'm trying to install Orocos on a Fedora Core 4 sytem patched with RTAI.
I installed RTAI and it is running ok. I've installed al the otther 
components needed by Orocos.
When I do ../configure --with-lxrt=/usr/realtime I get the following error:

chekcing which target OS you want to configure for ... LXRT
configure: error:
you must specify the location of your patched linux kernel headers when 
using Lxrt.
For example: --with-linux=/usr/src/linux-rtai

What am I doing wrong. I want to use LXRT for user mode. And the 
directory /usr/realtime exists. orocos is installed on /opt/orocos
Please help.

Kind regards,

Muhamber Eglence