Re-try Minisat download (up to two times) in case it fails

[tautschnig]

Running in CI we do see transient network access errors from time to time, which
should not result in task failures (as long as they are genuinely transient).

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[martin-cs]

It's a fudge but ... I can see why it's useful.

[smowton]

Nitpick: prefer $(cmd) to backticks

[chrisr-diffblue]

I can see the desire to have this, but I have the feeling it's possibly implemented in the wrong place... Would this actually be better done in the .travis.yml, etc? As it stands this also does nothing for CMake based builds, but I appreciate that they handle the minisat download in a different way which is probably more annoying to fix up.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: f03a2ca).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87498958

[peterschrammel]

I agree with @chrisr-diffblue that this should be done in .travis.yml. travis_retry cmd is probably the way to do it: https://docs.travis-ci.com/user/common-build-problems/#timeouts-installing-dependencies

[tautschnig]

My reason for not putting it in the Travis config file was to avoid duplication: the very same code would also go in the CodeBuild specs for both Linux and Windows and (at least in my use case) a number of build rules living outside the repository. All of which currently invoke make minisat2-download, so I figured I'd just put it right in the Makefile. Hence, unless I hear further objections, I'd plan to implement @smowton's suggestions but otherwise leave it in the Makefile. I am, however, happy to replicate this for CMake as well.

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: b223bfe).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87819761
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: cc4347d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87820352