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On modern FreeBSD ports, glut has been replaced by freeglut, which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.
add likely/unlikely branch hint macros

Provide likely/unlikely branch hinting macros.
core: don't crash if pc is past flash

If we somehow had bad code that pointed us past the end of the flash it
would segfault simavr rather than catching the bad case, this fixes and
that wraps the check in an unlikely() branch hint since its very
unlikely we will take that case.