Yeap, you are getting those errors because you don't have the adc.c and/or adc.h properly linked.
You need it in the makefile as you posted, but also linked in your code:
#include "global.h" // include global settings
#include "uart4.h" // include uart function library, includes buffer.h
#include "rprintf.h" // include printf function library
#include "timerx8.h" // include timer function library (timing, PWM, etc)
#include "a2d.h" // include A/D converter function library