By modifying the Kbuild file as described to exclude the wcb4xxp driver, which we do not require, we do not get the error described above, and the compilation completes successfully. dahdi _cfg now runs successfully, I have progressed to the Asterisk build and I can see the Dahdi cards from Asterisk.
Assuming this build continues to work once I get to the test stage, I shall mark this topic as resolved.
For future reference, how often do the OpenVox Dahdi drivers get updated? Do you see this issue being resolved in some future mainline dahdi-linux-complete build?