Improve OCaml script to check md5 checksums (eg. don't call md5/md5sum anymore)
Revert previous change to the configure: we don't need to check for the presence of md5/md5sum anymore.

This reverts cd5dc8faf6165184f0f1565572c6a9d8eea479eb and 09aa904d7ecacd93b46ed57cbe5cd35d4afa68d5

(and it's a better fix for #563)