you can try adding one more line below to hardcode this flag:
# If OCTAVE_ROOT is defined, include Octave in mangOH Yellow builds by default.
# ...but we don't care about this if we are cleaning.
ifneq ($(MAKECMDGOALS),clean)
ifdef OCTAVE_ROOT
yellow_%: OCTAVE ?= 1
else
$(warning ==== OCTAVE_ROOT not defined ====)
yellow_%: OCTAVE ?= 0
endif
endif
yellow_%: OCTAVE ?= 0