Don't install (ln -s) libGL.so and libGLU.so, they get already installed
by some other port.
We lose the ability to _compile_ linux programs with Mesa3 support by
specifying "-lGL -lGLU", but someone who needs to do this can use
"-lMesaGL -lMesaGLU" instead (or come up with a better solution).
Noticed by: pointyhat