cryptol: Add cvc4 Cask dependency

[jawshooah]

Left the caveats alone, since it's still important that the CVC4 binary is in your PATH (installed to /opt/local/bin by default).

Per discussion in #8492.

[vitorgalvao]

Actually, we likely can remove most of the caveat, just not the files_in_usr_local part. The warning it outputs should be clear enough, and the other line of the caveat is rendered useless via your depends_on.

[jawshooah]

Unfortunately not; the cvc4 binary is installed to /opt/local/bin, which is not in the PATH by default, so the warning still applies.

[tapeinosyne]

Note that we must wait for the next stable release, which will ship depends_on :cask, before removing caveats re dependencies.

This PR only adds the stanza (which is backward-compatible), but I would prefer not to merge multiple patches to the effect of depends_on :cask.

[rolandwalker]

The caveats should still be removed here. Configuration of CVC4 belongs in the cvc4 Cask, and is being added in #8591.

[tapeinosyne]

Cask dependencies are now available in the stable release. Merging; caveats to be removed in another PR.