don't require mcrypt

[MegaphoneJon]

I feel that civicrm/civicrm-core#12733 implies that mcrypt should no longer be a requirement for civi-download-tools - especially since this breaks on Ubuntu Bionic.

My only question is if we should go a step or two further:

• Should we disable installing mcrypt on 7.2 via PECL?
• Should we disable installing mcrypt on all systems?

I can add to this PR or file follow-ups, but I'd like to poll the crowd first.

[totten]

I guess the final disposition of encryption is still under consideration. Regardless, agree it's a good idea to soften the dependency.

From a buildkit perspective, we had a similar transition regarding PHP's mysql/mysqli extensions: the extension was required in many versions of Civi, but in newer PHP it went away, and it was no longer a requirement in newer Civi. The main difference is that mcrypt may still be required on newer versions to support data-migration. (We have not yet figured out a way to decrypt that data with any other crypto library -- some of the cipher or block params seem to be unusual.)

I'd vote to treat it like this:

• For basic civi-download-tools, the extension mcrypt is recommended but not required. Maybe even use the same trick as mysql check.
• For civi-download-tools --full, skip the extension mcrypt if the platform doesn't readily provide it.

[homotechsual]

mcrypt isn't an issue on Bionic from my testing.

[totten]

Thanks, @MegaphoneJon . Agreed with removing the requirement but wanted to add a softer recommendation for now, so I merged manually and added b452d68.