The 536EP is a chipset for a Winmodem, but will work nicely on Linux with the resulting Intel536.ko module.
I've never seen a /usr/src/linux-(version)-obj directory. Is this something your distro is doing? Looks like someone wanted to try to keep the object files out of the source code. While that's possible, it can causes troubles later (like this one.)
If you build/install from source , you'll get /lib/modules/(version)/build symbolic linked to /usr/src/linux-(version). Then, autoconf.h will be in the right place that the script expects. The "build" directory in /lib/modules/(version) is so you can later compile modules against the source code. I'm curious how they'd get built with it linked to '/usr/src/linux-18.104.22.168-4-obj/x86_64/default. From what you've said, there is no source there. Prehaps you could fix the symlink. You can always put it back if need should arise later on.
I don't have a /boot/vmlinuz.autoconf.h either. Or even a vmlinuz. I think these are RedHat-isms. This in itself shouldn't halt the build. Ignore it. It's been awhile since my Intel536 source directory was install-default, so prehaps I fixed the script to not mind this file. I can't remember. If the script refuses to go forward without that file, remove that part from the script. The important thing is kernel version = source that is installed version.
Now we run into one other problem. Last I checked, the latest Intel536 source on their site was this tarball, Intel-536EP-4.71.tgz, which will not compile and work with 2.6.16-2.6.17 (and I want to say 2.6.15's as well, but that was awhile ago since I ran that kernel.)
Hopefully you have a distro-fixed version that will work on 2.6.16 and up. I've not seen one, but if your build bombs, this is most likely what is happening. Many things changed in the kernel source, and the code needs to be updated for the module. I found some patches on the net and compiled them together, and hacked in a few very ugly fixes just to get it working (C is definately not my language). The biggest trouble it seems is the kernel does not want to export the required symbols due to the fact that the module code is labeled Proprietry, and it wants GPL to export the symbols. Likely large parts of the module would need re-writing.