fix makefile for building on linux
fix makefile for building on linux

This may break OS X PowerPC, but I don't have such a machine to test on
at the moment.

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