[Xenomai] RTNet installation error

[Артём Лобач]

 Hi! It's the first time I try to install RTNet for Xenomai-patched kernel. I installed OS with Xenomai from RTXI LiveCD.
>> # uname -a
>> Linux A-L2 3.8.13-xenomai-2.6.4-aufs #1 SMP Wed Apr 29 17:19:21 EDT 2015 i686 i686 i686 GNU/Linux

After "make menuconfig" in rtnet source and applying parameters next error appeared:
"checking for RT-extension... configure: error: *** RT-extended kernel not found in /lib/modules/3.8.13-xenomai-2.6.4-aufs/source"

What may cause this error?

Thanks in advance!
Artyom