Support looking up the AVR_ROOT in /usr/local/avr
Explicitly request libgl from pkg-config

On modern FreeBSD ports, glut has been replaced by freeglut, which
does not provide a pkg-config file. However, it will get picked up
by explicitly requesting libgl from pkg-config, since they are
installed in the same location by default. Additionally, libgl
should probably required anyway since we are make OpenGL calls.
Merge branch 'prototype-warning' into merged