By default, check the certificate of downloaded dependency archives.

Use --disable-certificate-check if you want to bypass that. See #408 for instance.

Partially fix #563
Export the presence of md5 or md5sum during the configure phase
Check the md5sum of downloaded archives.

This should fix #563

Remark1: this will break the compilation of hosts which don't have md5 or md5sum installed. I'll add that in the sources.

Remark2: I've writtent md5check in ML because md5 (on OSX) and md5sum (on linux) don't have the same command-line interface.
Configure fails if md5 or md5sum are not installed