Guard md5sum printf by DEBUG

[I-mikan-I]

The md5sum benchmark included printf calls which were not guarded by any flag, meaning that the benchmark relied on stdout and requires any platform to support linking against a functional libc implementation.

With the added #DEBUG guard, none of the benchmarks perform I/O.

[jeremybennett]

Hi @I-mikan-I

Thanks for the patch. This benchmark is a work in progress as a candidate for release 2.0. Eventually we'll remove references to printf altogether, but this makes the benchmark immediately usable.

I have captured this action as issue #177

Looks good to merge.