fix malformed variable reference in makefile
fix malformed variable reference in makefile

minor-jason/avian,lostdj/avian,marcinolawski/avian,MaartenR/avian,joshuawarner32/avian,MaartenR/avian,marcinolawski/avian,lwahlmeier/avian,minor-jason/avian,lostdj/avian,bgould/avian,bigfatbrowncat/avian-pack.avian,lwahlmeier/avian,ucdseniordesign/avian,bgould/avian,ucdseniordesign/avian,bgould/avian,bigfatbrowncat/avian-pack.avian,lwahlmeier/avian,lwahlmeier/avian,joshuawarner32/avian,lostdj/avian,bigfatbrowncat/avian-pack.avian,bgould/avian,dicej/avian,ucdseniordesign/avian,bigfatbrowncat/avian-pack.avian,minor-jason/avian,marcinolawski/avian,lostdj/avian,joshuawarner32/avian,dicej/avian,marcinolawski/avian,MaartenR/avian,dicej/avian,ucdseniordesign/avian,joshuawarner32/avian,MaartenR/avian,dicej/avian,minor-jason/avian