autoconf.h file missing Kernel-2.6.15?

[Marco <marco4ever@libero.it>]

Hi all,
I have installed Debian Etch on my computer.

I have compiled my custom kernel with the linux-source-2.6.15 package, but
now when I install the HSFMODEM driver I get this error message:

WARNING: missing file
/lib/modules/2.6.15/build/include/linux/autoconf.h
The cause of this is usually a missing or unconfigured
kernel source tree (and sometimes an incorrect directory or symbolic link).

What did I forget on my kernel configuration?

Could you help me?
Thanks

Marco