This is not an error in board_given.c; it is an error in your Makefile.
Does anyone remember the complaints everyone had about not being able to
use funcitons in <math.h> from lab 1? The reason was that the Makefile
for lab 1 did not specify the correct linker option to link against the
math libraries. For this lab, you are writing the Makefile yourself. In
order to link against the math library, you have to add the correct option
to the "LFLAGS=" line. You should already know what this is. If you've
forgotten, look through some of the early newsgroup postings that contain
"math.h" in the subject line.
Alexander