There is an error in that Makefile. That is a "simply expanded" assignment instead of a "recursively expanded" assignment. Try changing that line to:
INST_ARCH = \$(ARCHNAME)
It seems that ARCHNAME is not set at the time that line is read; changing it to a recursively expanded assignment should avoid the problem by expanding $(ARCHNAME) when INST_ARCH is substituted instead of when it is defined.
Your problem could also be that ARCHNAME is somehow getting set to the empty string, which is also wrong. You may want to submit a bug report when you find the solution to this; it seems to me that building on Windows is subtly broken at the moment.