Prefix with dashes (unordered list) and put one variable on each
line (to avoid future conflicts)
Done via the following one-liner:
> sh -c 'for i in $(make -C tests/sys/aio PROG=foo -VPROG_VARS:O); do printf "\t\t- $i\n"; done'
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division