removed obsolete CFLAGS setting from arraywalk makefile
introducing ThreadedBenchmark and SimpleBenchmark classes

arraywalk now implements the SimpleBenchmark interface, in the future it
should be changed to the ThreadedBenchmark interface when the latter is
reasonably stable
todo: ThreadedBenchmark still needs some work in the timing callbacks area,
and has some small todos noted in the source file itself
This patch also includes the necessary changes to the benchmark
makefile, including some cleaning up of said makefile.