Change flag for no output to MYRIAD_NO_OUTPUT.

The old flag was MYRIAD_BENCH, which was changed to MYRIAD_NO_OUTPUT.
Change dbgen to neglect comments columns.

Conditional logic for the myriad benchmark was added. When given the
flag MYRIAD_NO_COMMENTS the dbgen does not generate comments attributes.
Add custom make target for myriad.

Using the target `myriad-dbgen` compiles the dbgen with MYRIAD_NO_OUTPUT
and MYRIAD_NO_COMMENTS set.