Difficulty building RTAI linuxcnc from source

There is no chance. No one has messed with the FPGA. I'm the only person who has even handled the board.

And, as fas as I can tell no-one has done anything much with the Motenc driver either.
The last significant change was in 2012:
github.com/LinuxCNC/linuxcnc/commit/3d26...5363f1aeaaff3c4174b9

Are you running released LinuxCNC or compiling it yourself? (I have been assuming the former)

I am running released LinuxCNC, although I would love to be able to build it. Then I think I would have a better chance of tracking down this board issue and also be able to work on motion control, my real interest.
For fun, I edited my motenc_motion.hal file on my working machine, adding the "debug 5" line. Then I started LinuxCNC and the application failed, saying it had encountered an unknown command "debug."
My working machine is Ubuntu 4.4.3.

The "debug" command was introduced in 2020 and isn't available before 2.8.0.

On older systems you could do this at the command line to set the debug level

echo 5 > /proc/rtapi/debug

If your system has a /proc/rtapi/ then that should work

At the command line on the working machine I did

echo 5 > /proc/rtapi/debug

then I started LinuxCNC and did "dmesg" at the command line. Earlier I had also done "dmesg" at the command line after starting LinuxCNC. With the higher debug level, there were many more messages from RTAI but none of them dealt with the Motenc board or its type.

When I try to build, things seem to be going okay until "make," when I get a message "Makefile.inc must specify RTPREFIX and other variables. Stop."

OK, go to linuxcnc-dev/src

./autogen.sh
./configure
make
sudo make setuid

That ought to work.

Actually, you probably need to install a bunch of dependencies too:

Go to linuxcnc-dev

./debian/configure
dpkg-checkbuilddeps

Will give you a list of packages to install. I typically copy it and edit it to remove the options, versions and alternatives and use it after "sudo apt-get install"

When I do dpkg-checkbuilddeps I get a Lllllooooooonnng list of packages. Looking at the packages, they don't seem relevant to a LinuxCNC build.
I'd like to go back to the ./configure step in the high-level build sequence. When I do ./configure things seem to go okay until the end, when configure reports it is unable to find libmodbus, so (following the suggestion in the error message), I re-run configure with "--without-libmodbus". Then configure reports the same phenomenon, missing libusb, so I re-run configure with "--without-libmodbus --without-libusb-1.0" and then configure returns:

error: /usr/src/linux-headers-4.19.195-rtai-amd64/include/linux/version.h: No such file or directory
gcc: fatal error: no input files

Indeed when I go to check what's in /usr/src/ there's nothing. It's empty.

The huge list is everyting that is needed to build a LinuxCNC package including documentation). You might as well do it, even if some of them are not needed, it only takes about 5 minutes.

One of the dependencies in the list is probably linux-headers-xxx-rtai, and that will solve the version.h problem (and many others)

I did a video about how to build Linuxcnc and how to sort out the dependencies.