hmm, you are right, i didn't think sscanf would asign an empty string to the value and lib_prefix uses the same code, but it works for lib_prefix because the makefile specifies: lib_prefix = $(prefix)/lib/pike so there is always a value.
for include it sets: include_prefix = @includedir@ which could be empty.
setting includedir = $(prefix)/include/pike might do the trick.
greetings, martin.