How to add debug select when building linuxcnc source code?

I don't fully understand your question.

LinuxCNC has a few separate debug levels that might be set. They all control the debug level of different things.

There is an entry in the INI, but I suspect that you don't mean that.
linuxcnc.org/docs/2.8/html/config/ini-config.html#_emc_section

You can add a line of code in a relevant place:
linuxcnc.org/docs/2.8/html/man/man3/rtap...sg_level.3rtapi.html

(you used to be able to set this with echo 5 > /proc/rtapi/debug but something broke that).
Try "man debuglevel" for another way. (basically issue the "debuglevel" command in a terminal window when LinuxCNC is running)

And then there is the option of adding debugging at the kernel level:
github.com/LinuxCNC/linuxcnc/blob/2.7-buster/docs/rtfaults.txt

The last link in my reply above says where the -g flag should be inserted.