The problem is that this code is disabled. It requires someone to go in and manually enable it, it's likely that it's never been built or run by an illumos user, and the Makefile warns against enabling it.

leave debug code as is - it can help enabel it if you need debug. many places in different platforms to do it. with debug, we try to inject additional code, but existen can help identify places where it was and use it if needed.

for example: on DilOS we do separate userland components build with GDB flags and do it by separate Makefile.gnu - no conflicts with dmake.

Igor has a point.  Debug code like that can be helpful to have around.
Also, I think there's probably a much simpler way to fix this, which is to 

arrange that the debug.o produced has something in it that will generate some CTF.

It doesn't have to be anything that will actually be used.  It just has to make the .o non-empty.

I haven't tried, but I suspect that approach could fix this bug without nearly so much change.

we have dtrace to inspect things.

The code will be kept around, forever. All you have to do is look back through the history in whatever source code management system is in use.
Again, there is no evidence whatsoever that this debug code has ever been compiled, let alone used. It's just dead. In the event that debugging were required in this area, then one could resurrect the code or, more likely, inject custom debugging code relevant to the problem being investigated.