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.

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.

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."

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.

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