libunistring: fix builds pre-Mojave

[lukaso]

make check does not pass pre-Mojave.

I've reopened this PR which was based on this closed PR: #106654 because concerns about upstream changes have been removed (sorry I didn't have time to add to last PR before it was closed).

• [x] Have you followed the guidelines for contributing?
• [x] Have you ensured that your commits follow the commit style guide?
• [x] Have you checked that there aren't other open pull requests for the same formula update/change?
• [x] Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [x] Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• [x] Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

---

[Bo98]

Can you detail what system you had the issues on? I cannot reproduce the issue on 10.11 El Capitan.

[lukaso]

Can you detail what system you had the issues on? I cannot reproduce the issue on 10.11 El Capitan.

I was compiling against a 10.12 and 10.13 SDK. In both cases the compile failed.

[lukaso]

I think the test failed because it ran out of time.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.