Freebsd build fix

I had a few issues building on FreeBSD 9.2. The LINUX_AVR_ROOTS was not general enough since BSD uses /usr/local/avr for its avr-libc directory. Also, FreeBSD uses freeglut for its glut implementation. Unfortunately, freeglut doesn't provide a pkg-config file. As a workaround, I was able to pull in the gl package with pkg-config and explicitly pass -lglut and get it to link.